Houses fall down. Trees fall over. Large holes form in the ground. Could Alaska be sinking? That's what some villagers in Alaska have been asking themselves recently--and scientists think they know why. Warmer temperatures may be causing Alaska's frozen ground to thaw . About 85 percent of Alaska's land surface has  _ . Permafrost supports the ground above it--including trees, houses, and roads. But why is the permafrost thawing? Some scientists say that human-caused pollution is leading to global warming. And most scientists agree that Alaska has been getting warmer. They say that the warmer temperature is causing permafrost to thaw in some areas. The thawing permafrost can cause problems for people, plants, and animals. "The collapsing  is a problem in the villages in Alaska which are built on permafrost, and some of them may have to move to safer ground, where there is no permafrost," Weller said. The Alaskan town of Glennallen saw its old post office collapse. And, according to James Walters, a permafrost expert at the University of Northern Iowa, house-moving companies have been very busy. "Melting permafrost can also destroy trees and forests," Weller said. "When holes in the ground form, trees fall into them and die. This could turn a forest into a swamp , and animals which need the forest will have to move elsewhere." The melting could cause "severe problems." Weller says: "This could take hundreds of years, but in the end Alaska will look quite different from what it looks like now." The main reason why permafrost thaws is that   _  .
the temperature in Alaska is rising people in Alaska don't take care of permafrost too many houses are built on permafrost people in Alaska polluted permafrost