Cherskiy, RussiaNikita Zimov was teaching students to do ecological fieldwork in northern Siberia when he stumbled on a disturbing clue that the frozen land might be thawing far faster than expected.

Zimov, like his father, Sergey Zimov, has spent years running a research station that tracks climate change in the rapidly warming Russian Far East. So when students probed the ground and took soil samples amid the mossy hummocks and larch forests near his home, 200 miles north of the Arctic Circle, Nikita Zimov suspected something wasn't right.

In April he sent a team of workers out with heavy drills to be sure. They bored into the soil a few feet down and found thick, slushy mud. Zimov said that was impossible. Cherskiy,