Permafrost is ground that stays frozen solid year-round, even during summer. This layer of earth remains frozen for at least two consecutive years, and often for thousands of years.

You'll find permafrost in the coldest parts of the world: Alaska, northern Canada, Siberia, and high mountain ranges. In these places, only the top few inches of soil thaw during summer, creating a muddy surface layer called the active layer. But underneath, sometimes hundreds of feet down, the ground stays frozen like a giant ice cube that never melts.

Permafrost affects everything above it. Trees growing in permafrost regions often lean at crazy angles because their roots can't dig deep into the frozen ground. Buildings need special foundations that won't melt the permafrost beneath them: if the frozen ground thaws, structures can sink and collapse into mud.

Scientists worry about permafrost because it contains ancient plants and animals that froze thousands of years ago. As Earth's climate warms, some permafrost is beginning to thaw for the first time in millennia, which creates problems for the communities built on top of it and releases gases that had been locked in the ice.