We’ve gotten a lot of questions about bridges and tunnels: How do bridges stay up? How are tunnels built? How do they build bridges over water? How do they put tunnels underwater?

To answer these questions we’re staging a little friendly competition: bridges vs. tunnels!

We’ll find out how they’re built, where they’re found in nature, and what they may look like in the future. Plus: A visit from our friendly neighborhood robot.

SPOILER ALERT: Click here to learn more about this episode’s mystery sound.

Music in this episode by Podington Bear via Creative Commons.

Plus: Check out a tunnel boring machine in action!

Audio Transcript

Download transcript (PDF)

ALEX MARRIOTT: You're listening to Brains On, where we're serious about being curious.


MOLLY BLOOM: I'm your host, Molly Bloom. And here with me today is co-host Alex Marriott, who is 12. Hi, Alex.

ALEX MARRIOTT: Hello, Molly.

MOLLY BLOOM: We've gotten a lot of questions about bridges and tunnels.

ALEX MARRIOTT: How do bridges stay up?

MOLLY BLOOM: How are tunnels built?

ALEX MARRIOTT: How do they build bridges over water?

MOLLY BLOOM: How do they put tunnels under water?

ALEX MARRIOTT: To answer these questions, we're staging a little friendly competition-- bridges versus tunnels!

MOLLY BLOOM: We asked you to tell us which you like better, bridges or tunnels. And we'll be playing some of your answers throughout the show. We'll start with siblings Addison and Heath from Navasota, Texas.

HEATH: I like tunnels because sometimes it's underwater, and sometimes they're under the ground. You get to go under something.

ADDISON: I like bridges because they're up high, and sometimes water is below them.


ALEX MARRIOTT: Today, representing bridges, we have Producer Sanden Totten.

SANDEN TOTTEN: Thank you, thank you. Bridges all the way.

ALEX MARRIOTT: And representing tunnel's, Producer Marc Sanchez.


MOLLY BLOOM: Mark and Sanden have prepared some opening statements. Sanden, you won the coin toss. We'll start with you.

SANDEN TOTTEN: I can't fly. But when I drive over a really long tall bridge, I almost feel like I'm soaring.


The world is below me, undisturbed and awe inspiring. And I'm just gliding above it. This is the kind of thing normally you have to be either a bird or have super powers to experience. But bridges let us do this all the time.

Bridges are also often really, really pretty. A well-designed bridge, like the Tower Bridge in London or the Golden Gate Bridge in San Francisco, can be the symbol for a city. We take pride in our bridges. And our bridges take us all kinds of places.

They help us cross rivers, canyons, roads, bays, bayous, lakes, trenches, gorgeous swamps, straits, and much, much more. They save us hours of travel time, too. Why go all the way around something when you can just go over it on a bridge?

And if you're a construction nut, then bridges are totally for you. They take all kinds of cool equipment to build. They can be made from lots of materials. And each one is a little different from the next. But I think my favorite thing about bridges is that they bring us closer together.

Whether it's a highway connecting two towns across a bay or just a road over a river that lets you get to your best friend's house a little sooner, bridges help us stay connected. Plus, every time you high five, you're basically making a Friendship Bridge with your hands. Go on, try it.


ALEX MARRIOTT: Whoa, you're right.

MOLLY BLOOM: A rousing call for bridges, thank you, Sanden.

ALEX MARRIOTT: Next up, Marc, with some tunnel love.


MARC SANCHEZ: When I was a kid, I used to go to the beach with my parents. And besides splashing around in the ocean, the one thing I always did, the one thing that I could do for hours was to dig tunnels. There's something about scooping into the sand with your hands and feeling it rub against your fingers that is so satisfying.

I'd always start with a little hole. And after I'd get past the first layer or two of hot sand, I'd get to this sort of cool, damp sand. That's the stuff that makes for great tunnels at the beach. Within a few minutes, my hole in the sand would be so long, I could stick my entire arm into it.

But that's just one hole. My absolute favorite thing about digging around in the sand was after I started construction on the second hole, all of the sudden, the two holes would connect and instant tunnel. A virtual city of tunnels can be built in an afternoon.

A road that started on one side of the ice chest can come out of the other. Thanks, tunnels. If I was lucky driving to or from the beach, we would have to pass through an actual tunnel. And in my family, that meant two things.

First, [INHALE SHARPLY] we would see if we could hold our breath through the whole thing. And second--


--my dad would honk the horn. Driving through a tunnel is weird when you think about it. You're burrowing underground or you're speeding through a mountain. That is nuts! Time and space seem to change when you're in a tunnel, too.

The scene that you were watching from your window-- the birds, the trees, the airplanes-- all of it, gone-- well, at least for a minute. Things sound different in tunnels, too. There's lots of echo. And the lights, they're just rhythmically passing one after another, after another, after another.

Stay in a tunnel long enough, and you'll start to notice what they actually look like. Some have tile on the wall that look like shiny white eggs. Others are plain concrete tubes. I especially like the tunnels where you can see all the carved out rocks and soil.

And we're back. I drive through tunnels on my own these days. I am an adult after all. My daughter, and wife, and I have our own tunnel tradition. We take a deep breath, too. [INHALES DEEPLY] And for the entire length of the tunnel, we cheer--

FAMILY: Tunnel love!

MOLLY BLOOM: Ooh, that is fun.

ALEX AND MOLLY: Tunnel love!

SANDEN TOTTEN: OK, OK, it's fun to yell. We get it, we get it.


KIRA SOMERSET: Hi, my name is Kira Somerset. I'm eight years old, and I'm from Seattle, Washington. I like tunnels better because they go underwater. And they go through mountains. And when you honk your horn, there's an echo, echo, echo.

MOLLY BLOOM: In a lot of ways, tunnels and bridges are related. If you want to build either, then chances are good you're going to be a civil engineer.

ALEX MARRIOTT: Civil engineers are the people that design or build our roads, and bridges, and tunnels.

MOLLY BLOOM: And when we're stuck trying to figure out how to get from one place to another--

ALEX MARRIOTT: --should you put a road over some water--

MOLLY BLOOM: --or put one under it?

ALEX MARRIOTT: Civil engineers helped determine the best route.

MOLLY BLOOM: Sometimes that's above ground.

ALEX MARRIOTT: And sometimes you got to dig deep. Right, Marc?

MARC SANCHEZ: You bet. And now, we get to my friend, the tunnel. Some tunnels go through mountains. Others go underground. And some tunnels even go underwater.

To build an undersea tunnel, you drop big concrete tubes to the ocean floor. And once they're all connected, you drain the water out and voila-- tunnel. Well, there's a little more to it. But basically, that's what's happening.

MOLLY BLOOM: So that's how you build a tunnel underwater. So how do you build one above ground?

MARC SANCHEZ: I talked to Mike Mooney. He's a civil engineer who specializes in tunnels. You guys, he digs tunnels for a living. That's his job. Sometimes explosives are necessary to blast through tough rock. But when we're talking about digging tunnels, we're mostly talking about one device.

MICHAEL MOONEY: While tunnel construction mostly today is done with automated, semi-automated tunnel-boring machines--

MARC SANCHEZ: I have to stop Mike Mooney for a second here to say that tunnel boring machines, or TBMs, as they're sometimes called, are anything but boring-- terrible name if you ask me. You guys know what a food processor does, right? It chops food.

MOLLY BLOOM: So there's like a cylinder blade on top of it, and you can feed food into it, and it chops it?

MARC SANCHEZ: Yeah, exactly. You can push a potato down in there, and it makes really thin sliced potatoes.

ALEX MARRIOTT: Potato chips.

MARC SANCHEZ: You are correct-- and maybe a little bit hungry. That blade is about 7 or 8 inches across. Now, take that blade and make it super gigantic-- 60 feet in diameter, that's the biggest one right now-- and put it on a long metal tube. OK, back to Mike.

MICHAEL MOONEY: And these are large circular tubes with large cutting wheels on the front of them. They grind through the ground, churning up that material, and ingesting that material very much like an earthworm would.

MARC SANCHEZ: Each pass of the blade creates a mess of dirt. But have no fear, the TBM is prepared. Inside the long metal tube is a conveyor belt that takes the dirt and sends it back to the end of the tube.

MOLLY BLOOM: So if Mike is saying a TBM is like a big earthworm tunneling through soil, and you're saying the soil and rocks move through to the back end of the TBM, is it OK for me to think of tunnel-boring machines as giant metal earthworms that poop dirt?

MARC SANCHEZ: If that helps you, sure, giant metal earthworms that poop dirt. And at the same time the TBM is [CLEARS THROAT] pooping dirt, it also is building the walls of a tunnel.

MICHAEL MOONEY: The front part of the tunnel-boring machine, it moves ahead, say, 5 feet, 6 feet in one spurt, so to speak. And then simultaneously or after that excavation cycle, it will install a ring of concrete at the back end of the tunnel-boring machine. And then inch its way forward again.

So it's this sequence of moving forward 5 to 6 feet, placing a 5 to 6-foot ring of concrete behind it, and then continuing that process.

MARC SANCHEZ: So when the TBM finishes and digs out the other end, what's left is a series of concrete tubes-- basically, the skeleton of a tunnel.

MOLLY BLOOM: So that's the lowdown on digging tunnels. Alex, anything in there that you found particularly surprising or cool?

ALEX MARRIOTT: Well, tunnels that can go through mountains go practically through everywhere. And I think that's pretty amazing and cool.

ANDREW: Hi, I'm Andrew, and I'm 5. And I'm from Latrobe, Pennsylvania. The reason I like bridges more is because you can see more.

ALEX MARRIOTT: There are lots of different kinds of bridges.

SANDEN TOTTEN: My favorites are the BATS.

MOLLY BLOOM: Hey, Sanden. Welcome back. And aren't bats animals?

SANDEN TOTTEN: No. I mean, well, yeah, they are animals, yes. But BATS is also an acronym for some of the most common types of bridges-- beams, arches, trusses, and suspension-- B-A-T-S, BATS.

ALEX MARRIOTT: So tell us all about the BATS.

SANDEN TOTTEN: Well, let's start with the B, beam bridges. These are simple. So you just kind of lay a beam down over a gap.

MOLLY BLOOM: So like a log over a stream?

SANDEN TOTTEN: Yeah, totally. That's a very simple beam bridge. Highways often have beam bridges to cross small obstacles. So usually, the road is supported by two vertical columns.

And then you simply lay some beams horizontally across those columns, place your road on the beams, and suddenly, you've got a bridge. The longer the gap between those columns, though, the more those beams kind of sag in the middle, especially under heavy loads.

ALEX MARRIOTT: That doesn't sound too good for a roadway.

SANDEN TOTTEN: No, it's not. And so that's why we turn to sturdier bridges, like the A in BATS, arches. Now, for these, the deck, which is what we call the road on top of the bridge, the deck sits on what looks basically like a rainbow.

But the thing with Arch bridges is the longer they get, the harder they are to build. You have to make the supports bigger and bigger. And eventually, it's not even worth it.

MOLLY BLOOM: So to make a longer bridge, let me guess, that would be the T in BATS?

SANDEN TOTTEN: Exactly, the truss bridge. These were really popular back in the 1800s. Have you guys ever seen like an old Western movie, where a train goes over a really big bridge that has lots of steel or wood bars under it for support?

ALEX MARRIOTT: Yeah, I've seen several.

SANDEN TOTTEN: Think of that bridge. The structure under it is the truss. It's kind of like scaffolding. And if you look closely, the bars are set into a pattern of triangles.

MOLLY BLOOM: That makes sense. Triangles are one of the strongest structural shapes in the world.

SANDEN TOTTEN: Triangles are great. These bridges use that network of triangle bars to support the weight of the deck. Now, sometimes the truss is above the deck. And it kind of looks like a fancy tall railing along the side of the bridge.

These bridges, they can also only span so far before they start losing all their strength. So engineers invented another kind of bridge to go really far distances. And these are the S in BATS.

ALEX MARRIOTT: Suspension bridges.

MOLLY BLOOM: Like the Brooklyn Bridge in New York.

ALEX MARRIOTT: Or the Golden Gate Bridge in San Francisco.

SANDEN TOTTEN: Right, right. Here, the deck of the bridge is hanging from suspender cables attached to two very large monster cables. Those bigger cables are held up by towers. And when I say these cables are big, by the way, I mean they are really big, like several feet thick sometimes.

ALEX MARRIOTT: Wow. What have they made of?

SANDEN TOTTEN: Very strong steel. But if they were just solid steel, they would be way too stiff. And they wouldn't have the kind of bend and give that are needed for bridges. So the engineers came up with a really clever trick. Here's Reginald DesRoches, a civil engineer from the Georgia Institute of Technology, to explain.

REGINALD DESROCHES: Cables themselves are made of hundreds and thousands of tiny cables, which are millimeters in diameter that are branded together in the same way a rope-- if you carefully look at a rope, you see how a rope is branded together if you take it apart, same way you have these big cables that are branded together by small-- smaller wires. And they're strung together so that, in total, these things can carry a huge force themselves.

ALEX MARRIOTT: That's serious strength in numbers.

MOLLY BLOOM: Sanden, you mentioned that these cables for the suspension bridges are held up by two really big towers. I've always wondered, how do they build those towers when the bridge is over a large body of water, like an ocean?

SANDEN TOTTEN: Yeah, I used to think they just basically had construction workers in scuba gear who would just dive down there and build it all. But the real answer is even cooler than that. They use something called a cofferdam. So you guys know that cardboard tube at the end of a roll of paper towels?


SANDEN TOTTEN: So imagine you take one of those tubes, and you stand it up vertically in a tub of water. So the top of the tube is poking out of the water. Now, once this tube is in the water, you seal the bottom of it so no more water can get in.

Then you use a straw, and you suck the water out that's already inside the tube. Ta-da! You have a piece of dry land in the middle of your tub. That's basically what a cofferdam is-- except they can be cylindrical or square. And they are much, much, much, much, much, much bigger.

ALEX MARRIOTT: Big enough to build a tower in.

SANDEN TOTTEN: They're usually made of heavy steel and so large that barges or cranes have to put these cofferdams in place. And to make sure that these towers they build are stable, you really got to make sure you put this tower on solid ground even when you're building it underwater.

ALEX MARRIOTT: Wait, isn't the ground underwater usually all squishy and muddy?

SANDEN TOTTEN: Yeah, yeah. It's like my favorite part of going to the beach or a lake is a squishy ground you can get in your toes, which is great for squishing with your toes but not so great for bridges.

But here's what Engineer Andrew Ponzi told me. He works for the California Department of Transportation. And he says, to get through that muck, bridge builders actually use giant machines to pound in large beams called piles. These are kind of huge nails that'll eventually keep the towers anchored.

ANDREW PONZI: You nail the nail in the ground. And when it hits the hard spot, it's resting on it.

SANDEN TOTTEN: Now, if they can't reach solid ground, Andrew Ponzi told me, builders just keep pounding the piles into the ground until they get deep enough that they're held firm. Whatever the method, once you have those piles in place, you can build a cap on top of them and then set up your tower on top of that, nice and sturdy.

MOLLY BLOOM: Wow. So Alex, what do you think of that? How does that compare to our tunnel building?

ALEX MARRIOTT: Well, that's pretty amazing. I-- can't necessarily say that they're cooler than tunnels.

MOLLY BLOOM: OK So we're going to need another round for us to make up our minds about which is cooler, bridges or tunnels.

ARLINGTON: I'm Arlington from Magnolia, Arkansas. And I'm 6. I like tunnels because they go underground.

VIN: My name is Vin. I'm from Magnolia, Arkansas. And I like bridges because they go way, way, way up in the sky.


GROUP: Brains On!

MOLLY BLOOM: OK, guys, before we go any further, it's my turn to play something for you. It's time for the mystery sound.


CHILD: (WHISPERING) Mystery sound.

MOLLY BLOOM: Here it is.


Any guesses? Alex, what about you?

ALEX MARRIOTT: I'm guessing it's a train going through a tunnel.

MOLLY BLOOM: Excellent guess. Marc?

MARC SANCHEZ: Well, let's see. If we're talking about bridges and tunnels-- I don't know. [LAUGHS] I think it sounds like an alien.


MOLLY BLOOM: Sanden, do you have any guesses?

SANDEN TOTTEN: A whale love song maybe?

MOLLY BLOOM: OK, all excellent guesses.

MARC SANCHEZ: I should have said that.

MOLLY BLOOM: Well, we will be back with the answer in a little bit.


ALEX MARRIOTT: Do you have a mystery sound you want to share with us--

MOLLY BLOOM: --or a question--

ALEX MARRIOTT: --or a drawing?

MOLLY BLOOM: Send them to us at BrainsOn@M, as in Minnesota, PR.org.

ALEX MARRIOTT: We love getting emails from you.

MOLLY BLOOM: So how about getting emails from us?

ALEX MARRIOTT: Sign up for our newsletter at our website--

MOLLY BLOOM: --BrainsOn.org. We'll let you know about new episodes, events, and other fun stuff.

ALEX MARRIOTT: You can also keep up with us on Instagram and Twitter.

MOLLY BLOOM: We're @Brains_On.

ALEX MARRIOTT: And we're on Facebook, too.

MOLLY BLOOM: Now, is the time in the show where we honor the kids who keep the show going with their awesome ideas--

ALEX MARRIOTT: --marvelous mystery sounds--

MOLLY BLOOM: --and invigorating high fives. Here's the latest group of kids on the Brain's Honor Roll.



ROBOTIC VOICE: (SINGING) Brains honor roll. High five!

ALEX MARRIOTT: You're listening to Brains On from MPR News in Southern California Public Radio. I'm Alex Marriott.

MOLLY BLOOM: And I'm Molly Bloom. Today is all about a little friendly competition between bridges and tunnels.

ALEX MARRIOTT: And next up, our lightning round.


MC: But first, a word from your friendly neighborhood Robot.


ROBOT: You may hear some humans say "miles" when they're talking about a distance.

GROUP: Miles.

ROBOT: Or you may hear some humans say "kilometers."

GROUP: Kilometers.

ROBOT: They both measure distance. Kilometers are slightly shorter. 1.61 kilometers equals 1 mile. Or to put it another way, 1 kilometer equals 0.62 miles. You might be saying, yes, Robot. That's well and good. But what's the deal?

GROUP: What's the deal?

ROBOT: Well, most countries around the world use kilometers to talk about distance-- [SCRITCH] Afghanistan, Albania, Algeria, Andorra, Angola, Argentina, Armenia, Australia, Austria, Azerbaijan. I am told we do not have time for all the countries. But you have the idea.

Only a couple of countries use miles to talk about distance, mostly the United States and the United Kingdom. If you use kilometers, that's the metric system. If you use miles, that's the imperial system.

US GOVERNMENT: [SIGHS] We tried to switch to metric in the 1970s.

ROBOT: Sorry, US government. Your efforts did not work.

US GOVERNMENT: Well, we did make it voluntary. So--

ROBOT: Some humans consider the metric system a simpler system. For example, there are 12 inches in a foot and 5,280 feet in a mile. While in the metric system, there are 100 centimeters in a meter and 1,000 meters in a kilometer. Scientists often use metric measurements in their work, like grams, liters, and so on

GROUP: Science!

ROBOT: Back to you, Molly and Alex.

MOLLY BLOOM: Robot, want to stick around for the lightning round?

ROBOT: Yes, please. And thank you.

ALEX MARRIOTT: All right. Lightning round, commence.


ANNOUNCER: Lightning.

MARC SANCHEZ: Japan's Seikan tunnel is not only the longest continuous train tunnel at 33.4 miles--

ROBOT: 53.8 kilometers.

MARC SANCHEZ: --it's also the deepest. Almost half the trip is under the Tsugaru Strait. And it goes as deep as 240 meters.

ROBOT: 787 feet.

ANNOUNCER: Lightning.

SANDEN TOTTEN: The longest bridge? That, according to the Guinness Book of World Records, is the Danyang-Kunshan Grand Bridge in China. It's a train bridge over 100 miles long--

ROBOT: Over 160 kilometers long.

SANDEN TOTTEN: --farther than the distance from New York to Philadelphia.

ANNOUNCER: Lightning.

MARC SANCHEZ: The deepest tunnel for cars lies in Norway. It's called the Eiksund Undersea Tunnel. And it leads drivers 287 meters below sea level. That's a little over three football fields deep.

ANNOUNCER: Lightning.

SANDEN TOTTEN: The Millau Viaduct in Southern France takes the crown as the world's tallest bridge. It soars over 1,000 feet above the Earth, which is so high sometimes it's even above low-lying clouds.

ANNOUNCER: Lightning.

MARC SANCHEZ: We humans are not the only ones to go underground. Other tunnellers, you may have heard of include gophers, badgers, meerkats, prairie dogs, and-- my favorite-- the mole.

ANNOUNCER: Lightning.

SANDEN TOTTEN: Sure, lots of animals dig tunnels. But did you know that one species of ant can build bridges with their own bodies? Fire ants band together to form really resilient living bridges. Sometimes hundreds will cling together to make bridges across streams, which is icky to think about but awesome nonetheless.

ANNOUNCER: Lightning.

MARC SANCHEZ: There's a tunnel in Kuala Lumpur, Malaysia that does double duty. It's called the SMART tunnel. SMART, in this case, stands for Stormwater Management And Road Tunnel.

Most days, the SMART tunnel sees cars zooming through it. But when rains are heavy, the SMART tunnel is sealed off for traffic and turns itself into a tunnel for water. So instead of flooding the streets, water is sent underground and funneled into water treatment plants for reuse-- smart, indeed.

ANNOUNCER: Lightning.

SANDEN TOTTEN: In Japan, there's a bridge so advanced it has its own team of robots. The Akashi Kaikyo Bridge is made of steel, which can rust and cause problems. So the engineers built simple robotic painters that can travel to different parts of the bridge that are in danger of rusting and fix them with a new coat of paint. How about that, Robot, a new job for you maybe?

ROBOT: No, thank you. I have been programmed to be afraid of heights.

ANNOUNCER: Lightning.

SANDEN TOTTEN: Phew. Oh, man, that was tough. Nice job, Marc. Those were some cool facts.

MARC SANCHEZ: I know, you too. Bridges are kind of cool. [CHUCKLES] I mean, go tunnels.

SANDEN TOTTEN: No, no, bridges rule. But I guess tunnels are OK.

MOLLY BLOOM: All right, Alex. What do you think about all that-- team bridge or team tunnel?

ALEX MARRIOTT: After this, I think team bridge raised its status. But have you seen any bridges going underwater?

MOLLY BLOOM: That is a very good point. But we're going to talk about that in a little bit.

GROUP: (SINGING) Ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, ba, Brains On.

ALEX MARRIOTT: Marc and Sanden, one more round before the final verdict. What is the future of bridges and tunnels?

SANDEN TOTTEN: The future of bridges is all about adding smart sensors. Right now, you need humans to visually inspect a bridge for wear and tear. But Reginald DesRoches with Georgia Tech says, in the future, the bridge might be able to inspect itself.

REGINALD DESROCHES: More and more, we're building instrumentation that we can put on bridges that will tell us exactly what's happening on a bridge, how many cars are driving on a bridge, and when a bridge might be dangerously close to collapsing.

SANDEN TOTTEN: He says, one day, you might even have robots or drones that can fly up to a bridge and make critical repairs. So we could see the rise of smart self-repairing bridges in the future.

MARC SANCHEZ: OK, Sanden, interesting points. But did you know that in the coming years, tunnels will be essential, especially as our population increases? Right now, there are about 7.3 billion people in the world. And within the next hundred years, it's estimated that number will grow to over 11 billion.

Now, with all those people on top of the Earth, we're going to run out of space for buildings. Tunnel-boring machines keep getting bigger and bigger. And that's good. Our expert tunneller, Mike Mooney, says, there's plenty of room for big spaces underground.

MICHAEL MOONEY: Below ground is really the next frontier. So you can imagine large spaces for hockey arenas, and theaters, and parks that could all go underground. And these will require large spans and big spaces.

MARC SANCHEZ: After talking with Mike, I can really imagine walking out of my underground apartment, hopping on a subway that goes through a series of tunnels to, say, go see a big concert at an underground arena. So long, blue sky. I'm going to miss you.

ALEX MARRIOTT: You could just walk aboveground.

MARC SANCHEZ: Oh, yeah. I guess you're right. Thanks, Alex. Hello, sunshine-- and tunnels.

MOLLY BLOOM: So Alex, what do you think-- bridges or tunnels? Kind of seemed like a tie to me.

ALEX MARRIOTT: Well, I'm still voting for tunnels.

MOLLY BLOOM: You're team tunnels?

ALEX MARRIOTT: Yeah. But I'm pretty sure bridges someday can catch up--


ALEX MARRIOTT: --if they put bridges in underwater tunnels.

MOLLY BLOOM: [LAUGHS] I can't make up my mind. But I don't have to choose after all. Let's go back to that mystery sound.


Here's Mr. Robert Johnson with the answer.

ROBERT JOHNSON: That was the sound of one of the exhaust fans. And the fans are approximately 12 foot in diameter. And they spin fast enough to turn at about 260 revolutions per minute.

MOLLY BLOOM: These big fans that Mr. Johnson works with are used to clean the air inside the tunnel of exhaust from cars and trucks. But this isn't just any tunnel. This tunnel is special. It's part of the Chesapeake Bay bridge tunnel.

SANDEN TOTTEN: Wait, what? Did you say bridge tunnel?

MOLLY BLOOM: Yes, it's a bridge and a tunnel.

ROBERT JOHNSON: You have the Atlantic Ocean on one side and the Chesapeake Bay on the other side. And when you come to the first tunnel, it seems like the bridge is just disappearing. And you drop down from about 30 feet above sea level to approximately 90 feet below sea level.

And when you go into the tunnel, it's just like going into a circle of darkness because it's darker inside the tunnel than it is outside. But once you get into the tunnel, your eyes get accustomed to the lighting. And then you can just move right on through without any issues. And then once you come out of the tunnel, the first tunnel, you're back on to bridge again. And you're back to being 30 feet above sea level.

MOLLY BLOOM: They were originally planning to build just a bridge over this 20-mile stretch. But the Navy wanted to be able to move some really big boats through there, like aircraft carriers. So by putting part of it underwater, the ships are able to move through-- hence, a bridge tunnel. And let's face it, this friendly competition has shown that bridges and tunnels are basically opposite sides of the same coin.

MARC SANCHEZ: Yeah, I guess they're basically best buds, not mortal enemies.

SANDEN TOTTEN: Yeah, kind of like us, Marc. Go team bridge and team tunnel.

ALEX MARRIOTT: I love a happy ending.

MOLLY BLOOM: That's it for this episode of Brains On.

ALEX MARRIOTT: This episode was produced by Marc Sanchez, Sanden Totten, and Molly Bloom.

MOLLY BLOOM: Many thanks to Johnny Vince Evans, Joy [INAUDIBLE], Clement Marriott, Tom Weber, Eric Wrangham, Matty Mahon, Chrissy Pease, Sam Chou, Jim McGinn, Vicki Kreckler, Koko Sanchez, and Paige Addison. Music in this episode by Podington Bear. Check him out at PodingtonBear.com.

ALEX MARRIOTT: If you want more Brains On, head to our website or subscribe in your favorite podcast.

MOLLY BLOOM: If you're a fan of Brains On, consider leaving a review in iTunes.

ALEX MARRIOTT: It really helps kids and parents find out about the show.

MOLLY BLOOM: And you can always send questions, mystery sounds, drawings, and high fives to Brains On at M, as in Minnesota, PR.org.

ALEX MARRIOTT: Thanks for listening.


Transcription services provided by 3Play Media.