How Marie Tharp mapped the world's hidden floor

HostWe like to think we have the whole world mapped out by now, but for a long time, we knew more about the moon than we did about the bottom of our own ocean. Most people just assumed it was a flat, boring stretch of mud and nothing else.

HostThe person who finally proved them wrong was a researcher named Marie Tharp, but here is the thing: she wasn't even allowed to go out on the boats that did the measuring. How do you map a place you're literally forbidden from visiting?

GuestIt's a pretty wild story. Back in the nineteen fifties, Marie was working at Columbia University, but there was this old superstition that having women on research ships was bad luck. So she had to stay behind in the office while her partner, Bruce Heezen, went out to sea. He would use sonar to bounce sound waves off the bottom and measure the depth. When he came back, he just handed her these long, messy scrolls of paper with lines on them.

HostSo she's just sitting at a desk looking at these scribbles? That sounds like trying to draw a portrait of someone by only looking at their shadow.

GuestHonestly, it was even harder than that. She was looking at vertical slices of the ocean floor, basically one-dimensional lines of depth. To make a map, she had to use something called mathematical interpolation. Basically, she had to look at those scattered lines and use math and a huge amount of geological imagination to fill in the blanks. She had to figure out the three-dimensional shapes of mountains and valleys she would never actually see with her own eyes.

HostBut she was just guessing, right? If she's not there, and the data is just these thin lines, how could she know if she was drawing a mountain or just a bump in the mud?

GuestWell, she understood how landforms behave. In nineteen fifty three, while she was plotting these profiles of the North Atlantic, she saw something that changed everything. She noticed a deep, V-shaped notch right at the top of a massive underwater mountain range. She knew immediately that it was a rift valley. That's a place where the earth is actually being pulled apart.

HostWait, if the earth is pulling apart, that sounds like that old idea of the continents moving around. But I thought scientists back then thought that was a joke.

GuestOh, they hated it. A guy named Alfred Wegener had suggested the idea of continental drift years before, but the experts mocked him for it. They thought the earth was just a solid, cooling ball of rock. When Marie showed her rift valley to Bruce, he didn't want to hear it. He actually dismissed it as girl talk and made her redraw the maps. He was terrified that if they published something that supported continental drift, it would ruin their careers.

HostHe actually called it girl talk? That's infuriating. I mean, the data was right there on the paper. How do you just ignore a giant crack in the middle of the ocean?

GuestIt shows you how hard it's to change a mind once it's made up. But Marie didn't give up. To try and settle the argument, she and Bruce started doing something else. They took a map and started putting a dot everywhere there had been an undersea earthquake. When they finished, they found a perfect match. The earthquakes weren't just scattered all over the place. They were all concentrated right inside that narrow V-shaped notch she had mapped.

HostThat's incredible. It's like the earth was screaming exactly where the cracks were.

GuestIt really was the smoking gun. That match between the earthquakes and her rift valley forced the whole scientific community to accept plate tectonics. It proved that the seafloor was spreading and that the earth is a dynamic system of moving plates. It took a few more years for Bruce and the rest of the world to give her the credit she deserved, but she had fundamentally changed how we see our planet.

HostBut even then, most of us don't look at earthquake maps. We look at the world and see blue for the ocean and green for the land. How did she get the rest of us to see what she saw?

GuestShe changed the way we look at maps. In nineteen seventy seven, she worked with a painter to create a map of the world ocean floor. Instead of just a chart with a bunch of depth numbers, she made it look like a landscape painting of the world without any water. By choosing to show it in three dimensions, she made the hidden geology of the planet easy for anyone to see.

HostI think I have seen that map. It looks like there are giant scars running all the way down the middle of the sea.

GuestExactly. It revealed that the longest mountain range on Earth is actually underwater. It snakes around the whole globe like the seams on a baseball. Her work didn't just fill in a few blanks on a chart. It gave us the visual proof that the very continents we live on are constantly, slowly, on the move.

HostThe seafloor isn't just a quiet field of mud after all; it's the engine that moves the entire world.

GuestIt's a massive, shifting landscape that was invisible to us until one person decided to trust the math and the shapes on a page.

HostThose messy scrolls of paper and a bit of math ended up showing us a world that Marie Tharp was never even allowed to sail across.