A crater. Caldera are formed as the result of a volcano erupting and/or the top of the volcano collapsing. Most volcanos display the basic property of the collapsed vent at their peaks but unless this crater is particularly large (3-18 miles in diameter) it is considered a sink.

Crater Lake in Oregon has a six mile diameter caldera formed thousands of years ago. Some other kick-ass examples include Krakatoa in Indonesia (1883) and Katmai, Alaska in 1912.