Fairbanks, Alaska, United States
Permafrost is ground that stays frozen year round; the permafrost in interior Alaska also has massive wedges of actual ice locked within the frozen ground. When that ice melts, the ground surface collapses and forms a sinkhole that can fill with water. Thus, a thermokarst lake is born. At first glance, Big Trail looks like any lake. But look closer and there's something disturbing the surface: bubbles.