Cenote Angelita is comprised of fresh water until about 29 meters when it switches to a 1-meter layer of hydrogen sulfide, after which the entire cave bottom is filled with saltwater from 30 to 60 meters deep. So what appears as a river is actually a dense layer of saltwater resting at the bottom of a cave.
Cenote (pronounced, say-no-tay) is a natural sinkhole resulting from the collapse of limestone bedrock that exposes groundwater underneath. Especially associated with the Yucatán Peninsula of Mexico, cenotes were sometimes used by the ancient Maya for sacrificial offerings.
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 cave roof have collapsed revealing an underlying cave system, and the water flow rates may be much faster: up to 10 kilometers (6 mi) per day.
Cenotes around the world attract cave divers who have documented extensive flooded cave systems through them, some of which have been explored for lengths of 100 km (62 mi) or more. Cenotes are formed by 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. The rate of collapse increases during periods when the water table is below the ceiling of the void, since the rock ceiling is no longer buoyantly supported by the water in the void.
The Yucatán Peninsula contains a vast coastal aquifer system, which is typically density-stratified. The infiltrating rainwater floats on top of higher-density saline water. intruding from the coastal margins. The whole system is one that is land-locked but connected to an ocean.
Where a cenote, or the flooded cave to which it is an opening, provides deep enough access into the aquifer, the interface between the fresh and saline water may be reached. The density interface between the fresh and saline waters is a halocline, which means a sharp change in salt concentration over a small change in depth. Mixing of the fresh and saline water results in a blurry swirling effect caused by refraction between the different densities of fresh and saline waters.
In the Yucatán Peninsula the cenote depth is 10 to 20 m (33 to 66 ft) below the water table at the coast, and 50 to 100 m (160 to 330 ft) below the water table in the middle of the peninsula, with saline water underlying the whole of the peninsula.
In 2001–2002 expeditions in the Yucatan discovered three human skeletons; one of them was carbon-dated to be 13,600 years old. In March 2008, a dive team members of the Proyecto Espeleológico de Tulum and Global Underwater Explorers dive team, at a depth of 57 m (187 ft) located the remains of a mastodon as well as at 43 m (141 ft) a human skull that might be the oldest evidence of early man in the region.
The Yucatán Peninsula has almost no rivers and only a few lakes, and those are often marshy. The mny cenotes are the only perennial source of potable water and have long been the principal sources of water in much of the Yucatán Peninsula.
Major Maya settlements required access to adequate water supplies, and therefore cities were built around these natural wells. Some cenotes also played an important role in Maya rites. Believing that these pools were gateways to the afterlife, the Maya sometimes threw valuable items into them. The discovery of golden sacrificial artifacts in some cenotes led to the archaeological exploration of most cenotes in the first part of the 20th century. An American diplomat began dredging the Sacred Cenote there in 1904. He discovered human skeletons and sacrificial objects confirming a local legend, the Cult of the Cenote, involving human sacrifice to the rain god Chaac by ritual casting of victims and objects into the cenote.
More information as well as some eerie photos can be seen at the following sites.
Caves and Sink Holes
Go Cave Diving With Steve Bogaerts