All the land on Earth was one piece. You could walk from near the South Pole to the far north, and never once cross the sea. And deep in the middle of it, the rain stopped coming.
When all the continents join up like this, geologists call it a supercontinent. This one is Pangaea. The name means all the land. Here is the map you know. Lay it over the top, and every coastline is pressed against another one. There is no Atlantic. Around the whole thing runs a single ocean, and it covers most of the planet.
Building it took tens of millions of years. Pieces of crust drifted together. Every time two of them met, the collision pushed up mountains along the seam. Where the biggest pieces met, near the equator, the crust crumpled into a mountain chain. It may once have stood as high as the Himalayas. By two hundred and ninety-nine million years ago, the job was essentially done. A few narrow seaways were still open. Then those closed as well. Watch the last one go.
One enormous piece of land. And that turns out to be a problem.
Rain comes from the sea. Air over the ocean picks up water, carries it inland, and drops it as it goes. Pangaea's summers heated so much land at once that it pulled in sea air on a giant scale. It is called a megamonsoon. The coasts were soaked. But the air was dropping its water on the way. With every step inland, there was less left to fall. By the time it reached the middle, there was almost nothing. And the mountains made it worse. Air climbing over them dropped its last rain on the near side.
So life crowded toward the wet edges. Seed-fern forests grew along the coasts and up the mountain valleys. Four-legged hunters walked the river plains near the sea. One of the biggest was Dimetrodon, three meters long, with a tall sail along its back. Go inland, and all of it thinned out.
How do we know? The rock kept a record. This is salt. When a shallow lake dries out in the sun, the salt is what stays behind. Geologists call rock like this an evaporite. Rocks from this age are full of it, band on band. Every band is a lake that filled up, then dried to nothing. Now put it next to coal. Coal is a swamp forest, drowned and buried. Salt is water the sun took back. Coal says wet. Salt says dry. And across Pangaea, the salt kept winning.
And the longer the land stayed joined, the drier its heart became.
The middle of Pangaea is now the largest desert the planet has ever had. Dunes, red rock and salt flats, running on for thousands of kilometers. No river reaches it. Almost nothing lives here. It is farther from the sea than land had ever been.
It began with one strip of sea closing. It ended with the sea gone from the whole middle of the world. All the land in one place, and a desert where its heart should be.
