Top 10 Largest Known Underwater Caves

Located in Kentucky, USA, the Mammoth Cave System is the world's longest known cave system, spanning approximately 643 miles (1,035 km) in length. This vast network of underground passages and chambers was formed over millions of years through a combination of erosion, dissolution, and tectonic activity. The cave system features an estimated 400 miles (640 km) of mapped passages, with much of it still unexplored. The Mammoth Cave System is often referred to as the "world's largest cave" due to its immense size and complexity. The cave system was first explored by Native American tribes, who used it for spiritual and ceremonial purposes. Its unique geological features have been extensively studied in recent years.

Located near Kish Island in Iran, the Cheve L'AnCnoc is a vast underwater cave system estimated to be around 250 million years old. The cave's entrance is submerged at a depth of approximately 1,000 meters (3,300 feet), and it stretches for an impressive 8 kilometers (5 miles) beneath the seafloor. Its origins date back to the Triassic period, when it was part of a coastal plain. The exact location and full extent of the cave are yet to be fully mapped due to its remote location. Scientists have discovered unique geological formations within the cave.

Located in Belize, Central America, the Grand Blue Hole is an underwater cave system situated near the center of Lighthouse Reef Atoll at a depth of approximately 300 meters (1,000 feet). This natural wonder features massive crystal-clear waters and visibility up to 40 meters (131 feet). Its size and location have made it one of the most popular destinations for scuba diving and exploration. The Grand Blue Hole is estimated to be around 100 meters (330 feet) in diameter.

Located on the island of Madagascar, Tsingy de Bemaraha is a massive limestone karst formation known as "tsingy," with numerous sinkholes and canyons carved out by erosion over millions of years. This underwater cave system was first discovered in 1936 and has since been extensively explored. Its unique geology forms an immense network of sinkholes, canyons, and limestone walls. The Tsingy de Bemaraha is approximately 20 kilometers (12 miles) long.

Located off the coast of Japan, the Devil's Sea Cave system spans an approximate area of 10 square kilometers (3.9 sq mi). Its entrances are submerged at depths ranging from 30 to 400 meters (100 to 1,300 feet), with a central passage known as the "Sarushima Strait" approximately 8 miles long and 250 yards wide. This cave system was first discovered in 1967 by Japanese fishermen.

Located off the coast of France, the Le Passage underwater cave system stretches for about 3 kilometers (1.9 mi) at a depth of approximately 50-60 meters (164-197 feet). Its entrances are submerged and have been visible above water only on very rare occasions due to tidal changes. The exact location is difficult to pinpoint, but researchers believe it may be related to an underwater fault line.

Located off the coast of Canada, specifically at the northern tip of Newfoundland Island in Canada, the L'Anse aux Meadows is one of the largest and most well-known underwater cave systems known, although not as deep as some other entries on this list. This ancient Viking site was discovered in 1960 by Norwegian explorer Helge Ingstad.

Located off the coast of Greece, the Mavrovo Cave system is located near Mount Mavrovolos. Its entrance lies submerged beneath the sea surface at depths ranging from 15 to 25 meters (49-82 feet). Research has not indicated its full extent or any notable features.

Located off the coast of Thailand, this cave system consists of an impressive arch known as the "Kraba Arch," which stretches approximately 1.5 kilometers (0.93 miles) in length at a depth of about 30-40 meters (100-131 feet). Researchers are still working to map its full extent due to its remote location.

Located off the coast of British Columbia, Canada, the Thetis Cave system is approximately 400 meters long and features numerous small chambers and passages. This cave was discovered in 1959 by a team of researchers led by John Blashford-Snell.