Narrator: This is Science Today. Our planet is like a pot of boiling stew: thick stuff on top -- the continental plates -- and liquid below -- the mantle. The mantle is actually rock, not liquid, but over millions of years it rolls and boils, driving the continents around on the surface in a process called convection. But geologists know that something isn't quite right: according to conventional physics, the continents are too large, and there are too few of them.

Richards: I would expect, probably, a planet with three to four times as many plates as we have, if plates were representing kind of a normal convecting system.

Narrator: Geophysicist Mark Richards of the University of California, Berkeley and his graduate student Hans-Peter Bunge made a startling discovery:

Richards: The fact that the upper part of the mantle is runnier, if you will, than the lower part.

Narrator: According to their supercomputer model of the earth, about 1,000 kilometers down, the mantle suddenly gets 30 times thicker. It's as if, halfway down the pot, the stew suddenly became as thick as mud. Richards can't explain why -- but his model jibes perfectly with our actual planet. For Science Today, I'm Steve Tokar.