While there is great potential for cave diving in the continental karst throughout Mexico, the vast majority of cave diving in Mexico occurs in the Yucatán Peninsula. While thousands of deep pit cenotes (In the Yucatán Peninsula, any surface opening where groundwater can be reached is called cenote, which is a Spanish form of the Maya word d’zonot) are found throughout the Yucatán Peninsula, including the Yucatán and Campeche states, the renown extensive sub-horizontal flooded cave networks are essentially limited to a 10 km wide strip of the Caribbean coastline in the state of Quintana Roo extending south from Cancun to the Tulum municipality and the Sian Ka'an Biosphere Reserve, although some short segments of underwater cave have been explored on the north-west coast. The cave systems formed as normal caves underwater, but upper sections have drained becoming air filled during past low sea levels. During this vadose, or air filled state, abundant speleothem, mineral deposits formed from groundwater in underground caverns, deposits formed. The caves and the vadose speleothem were subsequently reflooded and became hydraulically reactivated as rising sea levels also raised the water table. These caves have therefore experienced more than one cycle of formation below the water table; they are polygenetic. Polygenetic coastal cave systems with underwater speleothem are globally common, with notable examples being on the Balearic Islands (Mallorca, Menorca) of Spain, the islands of the Bahamas, Bermuda, Cuba, and many more. As with all cave speleothems, the underwater speleothems in the Yucatán Peninsula are fragile. If a diver accidentally breaks off a stalactite, an icicle-shaped mineral deposit, from the ceiling or any other speleothem formation, it will not reform as long as the cave is underwater so active cave conservation diving techniques are paramount. In plan form, the Quintana Roo caves are extremely complex with interconnected passages. When cave diving through the caves, the pathways appear to have many offshoots and junctions, thus requiring careful navigation with permanent tees or the implementation of jumps in the guideline. The beginning of the 1980s saw the first U.S. cave divers come to explore the cenotes, such as Carwash, Naharon, Maya Blue, resurgence rivers, such as Rio Mante, and sinkholes such as Zacaton. The last 30 years have also seen the discoveries of the Dos Ojos, Nohoch Nah Chich cave systems, underwater caves, such as Aereolito (the 5th biggest underwater cave in the world), Chacdzinikche, Dzibilchaltun, Karkirixche, and Ox Bel Ha (the longest underwater cave system at the time, and as of January 2013, it includes a 242km underwater passage (see QRSS for current stats)), and deep sinkholes, or pit cenotes, such as Sabak Ha, and Utzil. To this day, however, the deep caves of the central Yucatán remain largely unexplored due to the sheer number of cenotes found in the State of Yucatán, as well as the depth involved that can be only tackled using technical diving techniques or rebreathers. In the end of the last millennium closed circuit rebreather (CCR) cave diving techniques were employed in order to explore these deep water filled caves. The past 20 years have been marked with the implementation of new technology to explore previously unreachable underwater venues. By the late 1990s, technical diving and rebreather equipment and techniques had become common place. The use of hand held GPS technology and aerial and satellite images for reconnaissance during exploration has become common. New technology such as rebreathers and diver propulsion vehicles (DPVs) have become available and are utilized for longer penetration dives. Active exploration continues in the new millennium. Most cave diving exploration is now conducted on the basis of "mini projects" lasting 1–7 days throughout the year. From 2006, a number of large, previously explored and mapped cave systems have been connected utilizing sidemount cave diving techniques and no-mount cave diving techniques in order to pass through these tight cave passages, thus creating the largest connected underwater cave system on the planet, Sac Actun, which presently has a length extent of 220 km (See QRSS for current statistics). Many cave maps have been published by the Quintana Roo Speleological Survey (QRSS).