But as awesome as these craters are, it is also entirely possible that they were caused by something slightly less dangerous than methane gas (or alien invasions). Last time around, some people suggested that the craters were pingos, crater-like formations that appear in the frigid regions of the world. Pingos occur when water seeps into the permafrost and freezes. Because water expands when it freezes, the ice pushes the overlying soil up into a mound. If the ice melts when warmer temperatures arrive, the soil sinks down into the space where the ice used to be, creating a crater.