What it is
All the land on Earth in one piece. You could have walked from near the South Pole to the far north without crossing the sea. When all the continents join up like this, geologists call it a supercontinent. The name means all the land. Around the whole thing ran a single ocean, and it covered most of the planet.
How it was built
Pieces of crust drifted together over tens of millions of years. 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 that may once have stood as high as the Himalayas. The Appalachians are what is left of part of it. By about 299 million years ago, the job was essentially done. It began breaking apart about 200 million years ago.
Why it matters
Rain comes from the sea. Air carries the water inland and drops it on the way. Pangaea's summers heated so much land at once that they pulled in sea air on a giant scale, a megamonsoon that soaked the coasts. By the middle, there was almost nothing left to fall. So the middle became the largest desert the planet has ever had, farther from the sea than land had ever been.
How we know
The rock kept a record. Rocks from this age are full of salt, band on band. Every band is a lake that filled up, then dried to nothing. The joined continents left clues too: the same fossil plant, Glossopteris, turns up in South America, Africa, India, Australia and Antarctica, lands that are oceans apart today.