sinkhole

Sinkholes, also known as cenotes in Spanish, occur naturally when water slowly dissolves bedrock (usually limestone or gypsum, which erode easily) until the ground collapses and a hole is created. Because rock erosion first happens underground, leaving a top layer intact, the collapse of that layer often happens quite suddenly. Water can tend to collect in the hole, thereby creating even more of a hazard for the unaware. Sinkholes can be found all over the world and vary greatly in size, ranging from a few feet to hundreds or more feet in diameter, and from 1 to 100 or more feet deep.