So first some background: the Earth's interior has a mostly solid core at the center, a mantle that is made up of flexible heated rock and a very very thin layer (five to 30 miles thick) of solid rock at the edges of the planet. That crusty outer layer is called the Earth's crust. During cold periods in the Earth's history, ice starts building up on land. In large amounts, this ice becomes glaciers and ice sheets, building up into vast masses thousands of feet thick. Greenland and Antarctica have the last two significant ice sheets on Earth. With that much of a load on top of it, the crust sinks into the mantle. But when the ice melts away or gets thinner during warmer periods, the crust starts to bounce back, a process called postglacial rebound.