The world is filled with but perhaps none hold as much mystery as the earth's blue holes, large marine caverns and water-filled sinkholes. Blue holes, which extend below sea level for most of their depth and may provide access to submerged cave passages, offer some of the world's most . These under-explored wonders were named for the contrast of the dark, deep blue color of their depths and the lighter color of the shallow water around them. In the Bahamas, which has the highest concentration of blue holes in the world, they have provided a great place for fossil discovery. Extinct animals, archaeological artifacts, and human remains .

Here, we take a look at the world's most fascinating blue holes in the Bahamas and beyond.

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1. Great Blue Hole, Belize

Located off the coast of Belize, the Great Blue Hole lies near the center of Lighthouse Reef and is part of the larger Belize Barrier Reef Reserve System, a UNESCO World Heritage site. The underwater sinkhole, which is over 400 feet deep, is a diver's dream—it's considered one of the top 10 diving sites in the world. Divers from all over the world travel here to dive in crystal-clear waters and see myriad species of marine life including, including Midnight Parrotfish and Caribbean reef shark, and spectacular coral formations.

2.Blue Hole, Dwerja, Gozo, Malta

Malta's Blue Hole, a circular formation of limestone rock,is located on the small island of Gozo in the Mediterranean Sea. One of the most popular diving sites in the Maltese Islands, the Blue Hole overlooks and starts in a 32 foot-wide inland sea pool which leads into a large crevice and through to the open sea. Divers here can find such as octopus, starfish, fireworm, cardinal fish,barracuda, and bullet tuna, according to AFAR magazine.

3.Blue Hole, Dahab, Egypt

Located in east Sinai, near Dahab, Egypt on the coast of the Red Sea, the Blue Hole is a submarine sinkhole, around 300 ft deep. It has a shallow opening known as "the saddle," opening out to the sea, and a tunnel, known as "the arch."The site is dived almost every day by recreational divers. However, it has been called the "World's Most Dangerous Dive Site" due to several diving fatalities. Accidents happened as divers tried to find the tunnel through the reef connecting the Blue Hole and open water at about 170.6 feet depth.

4. Dean's Blue Hole, Long Island, Bahamas

At more than 600 feet deep, Dean's Blue Hole in Long Island, the Bahamas, is said to be the world's deepest blue hole. It is almost twice as deep as any other explored cave in the Bahamas, according to the Bahamas Journal of Science. Experts who have dived the site report that it is and on the fourth side by a turquoise lagoon and powder white beach, according to Bahamas GeoTourism. There is never any swell or waves inside the hole, and visibility is usually between 50-100 feet.Dean's Blue Hole, named after the family name of local landowners, is also the location of the annual International Free Diving competition. In 2007, William Trubridge set a world record in unassisted free diving here, with a dive time of 3’09” and without the assistance of fins, ropes or weight belts, Trubridge reached an unassisted free dive depth of 82 meters (around 269 feet), according to World Record Academy.

5. Blue Holes of Palau

The Blue Holes of Palau, part of the Micronesia region in the western Pacific Ocean, and descend to a large cavern that exits on the outer wall at about 82 feet, according to Lonely Planet.

6. Santa Rosa Blue Hole, New Mexico

An underwater cave connected to a deep lake in Santa Rosa, New Mexico, the Santa Rosa Blue Hole is known for its dangerous depths. Measurements in the 1970s suggest that the , but it's not known how far back it goes, or if it gets deeper, according to LiveScience. In 2013,team of spent a week attempting to explore the Santa Rosa Blue Hole, where they discovered everything from Mayan relics to the skull of Antillothrix bernensis, an extinct species of New World monkey once found on the Dominican Republic.

7.Watling's Blue Hole, San Salvador, Bahamas

Watling's Blue Hole in San Salvador is one of the many blue holes of the Bahamian islands. Despite its location in the middle of a rainy tropical island, Watling’s Blue Hole has no fresh water indicating that the hole is connected to the ocean by an underground tunnel.

8. Blue Holes of Abaco, Bahamas

Abaco Island, the northernmost island in the Bahamas, is home to several blue holes, which have lured divers from all over the world.The inland blue holes generally reach , and may contain cave passages that extend off of the walls or floors, according to Advanced Diver Magazine. Some of the inland blue holes found in the island include Nancy's Blue Hole, located in a mangrove swamp; Far Side Blue Hole (also known as Magical Blue Hole), which has waters so clear that divers feel as though they are floating in air; andSawmill Sink, a solution cave currently off limits to divers for research purposes.

9. and 10. Blue Holes of Andros, Bahamas

The Bahamas is home to many spectacular blue holes, but Andros Island is the "holy grail" location of blue holes for divers, according to many Bahamians. The island has the in the world, according to the Bahamas tourism board, with 178 blue holes on land (such as Guardian Blue Hole and Captain Bill's Blue Hole) and with at least 50 in the sea.They are home many unusual and unique cave fish and invertebrates, some not found anywhere else in the world.

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