Given a scientific passage and an answer, generate a question for the given answer.

Passage: The ice ages were very recent as Earth history goes. The animals are familiar, but they're still very different from modern ones. For one thing, most of them were huge. A small surface area-to-volume ratio is better for keeping warm (less surface to lose heat from). Although the dominant animals were mammals, the Pleistocene Earth still does not look quite modern. Answer: water
The mars rover collected round clumps of crystals that, on earth, usually form in what?