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Interesting, I didn't know any of this stuff. Relevant clip:

"Mercury, on the other hand, has only one solid shell for a crust, rather than Earth's many plates that shift about. As Mercury's molten iron core has cooled over the billions of years since the planet formed, it has contracted and the shell of rock surrounding it has cracked and shifted to accommodate the smaller size. Today, the signs of those changes are written all over the face of Mercury. "Some of these things are really, really big," said Paul Byrne of the Carnegie Institution of Washington and the Lunar and Planetary Institute. "There are some truly gargantuan cliffs on Mercury." Bryne is the lead author of a paper presenting the results in the March 16 issue of the journal Nature Geoscience. The greater shrinkage corresponds to a model of the planet with a much larger iron core, said McKinnon."

I'd love to see an artist's rendering of what those gargantuan cliffs must look like, says the guy who's afraid of heights.