Cenotes are natural features which pepper the limestone shelf known as the Yucatan Peninsula. Water dissolves away the limestone from underneath, causing a sinkhole. Eventually the sinkhole breaks through the surface, usually with a rather small opening revealing a large cavern underneath that's been in the making for thousands of years. The older the cenote is, the larger the hole becomes. Many cenotes have had their roofs completely eroded away over time, more resembling an abandoned quarry than a cave.