How water and land split the Earth's surface

HostIf you were to flatten the Earth into a smooth, perfect ball—shaving down every mountain and filling in every deep canyon—the entire planet would be buried under two and a half miles of water. It sounds like a total water world, but we spend our lives on dry ground. I want to look at how this split between land and water actually works and why it's such a close call. How did we end up with any dry land at all?

GuestWe really are just lucky. When you look at those famous photos of Earth from space, the planet looks like a blue marble. That's because about seventy-one percent of the surface is covered by water. Land only takes up about twenty-nine percent. But that ratio is a bit of a freak accident. If the Earth were just a little bit flatter, or if there were significantly more water in the system, the continents would be completely underwater. We would be living on a planet with no land at all.

HostBut seventy-one percent is still most of the map. It feels like water is the main thing here.

GuestOn the surface, it is. But that's kind of a trick. Even the deepest parts of the ocean are only a few miles down on average. Compare that to the distance to the center of the Earth, which is nearly four thousand miles. If the Earth were an egg, the water would be much thinner than the shell. It's a vanishingly thin film of moisture clinging to a giant ball of rock. We think of the oceans as huge, but they're really just a damp coat on the outside of a very dry stone.

HostWait, a damp coat? That doesn't sound right. If I stand on the beach, the ocean looks like a massive amount of stuff. You're saying it's basically nothing?

GuestIn terms of the whole planet, yeah. Think of it this way. If you took all the water on Earth—every ocean, river, and ice cap—and rolled it into one single drop, that drop would only be about eight hundred and sixty miles wide. That's roughly the distance from Salt Lake City to Topeka. That tiny ball represents every bit of water we have. When you put that next to the rest of the planet, water makes up less than point zero five percent of the total mass. The vast majority of the bulk is the rocky middle and the iron core, which have almost no liquid water at all.

HostThat really changes the picture. But I have heard there's more water hidden away, deep down. Is that just a myth about giant underground caves?

GuestIt's not caves, but the water is there. Scientists who study rocks have found evidence that there might be three times more water trapped deep in the middle layer of the planet than in all the surface oceans combined. But it's not liquid. It's not like an underground sea you could swim in. Instead, the water molecules are actually locked inside the crystal structure of rocks like ringwoodite, hundreds of miles down.

HostHow can a rock hold water like a sponge if it's solid? That sounds like a contradiction.

GuestIt's all about how the rocks act under massive heat and pressure. The water becomes part of the rock itself. And it's a good thing it's there. That deep water acts like a vital lubricant. It softens things just enough so the giant plates of the crust can slide around and recycle themselves. Without that hidden water, the crust would probably lock up and stay still. We wouldn't have the moving plates that keep the planet a living place.

HostSo the land we walk on actually depends on that hidden water to keep it moving?

GuestIt does. And the balance between the land and the water is always shifting. It's not a fixed number. It's just a snapshot of a moving target. If you go back to the last Ice Age, so much water was locked up in giant glaciers that the sea level was four hundred feet lower than it's now. There were huge land bridges connecting the continents. On the other hand, if all the ice melted today, the land would shrink quite a bit. Over a much longer time, volcanoes are always spitting new land out, and sinking plates pull water back down into the deep earth. This keeps a balance that prevents the Earth from becoming a total desert or a total water world.

HostThe land under our feet feels so permanent, but it's really just whatever is sticking out of the film at the moment.

GuestThe real shocker is that ringwoodite. Finding out that the biggest ocean on Earth is actually trapped inside solid rock hundreds of miles beneath our feet changes everything we thought we knew about our wet planet.

HostThe map looks different now, knowing that seventy-one percent of the surface is just a thin layer of blue paint over a world of dry stone.