A cenote ( or ; ) is a natural pit, or sinkhole, resulting from the collapse of limestone bedrock that exposes groundwater. The regional term is specifically associated with the Yucatán Peninsula of Mexico, where cenotes were commonly used for water supplies by the ancient Maya, and occasionally for sacrificial offerings. The term derives from a word used by the lowland Yucatec Maya——to refer to any location with accessible groundwater. The Yucatán Peninsula has an estimated 10,000 cenotes, water-filled sinkholes naturally formed by the collapse of limestone, located across the Yucatán Peninsula, in Mexico, some of them are at risk from the construction of the new tourist Maya Train. Similar rock-sided sinkholes like cenotes are common geological forms in low-altitude regions, particularly on islands, coastlines, and platforms with young post-Paleozoic limestone with little soil development. The term cenote has also been used to describe similar karst features in other countries such as Cuba and Australia. Definition and description Cenotes are surface connections to subterranean water bodies. While the best-known cenotes are large open-water pools measuring tens of meters in diameter, such as those at Chichen Itza in Mexico, the greatest number of cenotes are smaller sheltered sites and do not necessarily have any surface exposed water. Some cenotes are only found through small <1 m (3 ft.) diameter holes created by tree roots, with human access through enlarged holes, such as the cenotes Cenote Choo-Ha, Tamcach-Ha, and Multum-Ha near Tulum. There are at least 6,000 cenotes in the Yucatán Peninsula of Mexico. Cenote water is often very clear, as the water comes from rain water filtering slowly through the ground, and therefore contains very little suspended particulate matter. The groundwater flow rate within a cenote may be very slow. In many cases, cenotes are areas where sections of the cave roof have collapsed revealing an underlying cave system, and the water flow rates may be much faster: up to per day. The Yucatan cenotes attract cavern and cave divers who have documented extensive flooded cave systems, some of which have been explored for lengths of or more. Geology and hydrology Cenotes are formed by the dissolution of rock and the resulting subsurface void, which may or may not be linked to an active cave system, and the subsequent structural collapse. Rock that falls into the water below is slowly removed by further dissolution, creating space for more collapse blocks. Likely,