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What is a Blue Hole? Great Blue Hole ~ Dean's Blue Hole ~ The Blue Hole ~ The Blue Cave Korcula~ El Zacatón Sinkhole

Updated on July 30, 2012
The Great Blue Hole Located in Belize
The Great Blue Hole Located in Belize | Source

What is a Blue Hole?

Blue holes, generally located in low-lying coastal areas, can be inland caves or underwater sinkholes. Most blue holes are approximately 330-390 feet (100-120 meters) deep, with a roughly circular entrance having an approximate diameter of 82-115 feet (25-25 meters) across.

The world's deepest blue hole - at 1,100 feet (335 meters) - is located in Tamaulipas, Mexico. This blue hole, the El Zacatón sinkhole, is an inland blue hole.

Blue holes fall into two classifications: anchialine caves (inland blue holes) and submarine caves (ocean blue holes). Although the anchialine caves are inland they also contain saltwater.

Because of the unique water conditions found in blue holes, skeletal remains of extinct as well as still living species are well preserved.

Blue holes are somewhat circular in shape, steep-walled, and can be extremely deep. One Bahamian blue hole exceeds 600 feet (180 meters). These caves can also contain miles and miles of lateral passageways extending in any direction.

Blue holes derive their name from their appearance – the depths of the hole appear a deep dark blue while surrounded by the lighter blue shallows.

There is very little water circulation in a blue hole; and, it becomes anoxic (without oxygen) at the deeper depths. This lack of circulation and lack of oxygen makes a blue hole inhospitable to most sea life; however, bacteria flourishes in this environment.

Most inland blue holes contain both freshwater and saltwater. The freshwater will cover the top of the blue hole with the saltwater filling the lower depths. At the point where the freshwater and saltwater meet, the halocline, a corrosive reaction that is able to eat away at the rock takes place. As the rock is eaten away, lateral passages are created along the side of the vertical wall. Over time, these passages can travel great distances. The Sawmill Sink in the Bahamas has lateral passages that extend over 2,000 feet (600 meters).

Coral found in Great Blue Hole
Coral found in Great Blue Hole | Source

The Great Blue Hole in Belize

The Great Blue Hole, located in the Lighthouse Reef Atoll near Belize City, was discovered by marine conservationist Jacques Cousteau. This circular hole is approximately 1,000 feet (305 meters) across and approximately 400 feet (123 meters) deep.

Huge stalactites and stalagmites formed inside this blue hole during the ice age are a treat for divers from all over the world.

The video below shares a lot of information about the Blue Hole in Belize.

Dean’s Blue Hole in the Bahamas
Dean’s Blue Hole in the Bahamas | Source

Dean’s Blue Hole in the Bahamas

Dean’s Blue Hole is the deepest sinkhole whose entrance is below sea level known to man. At 663 feet (202 meters) deep, Dean’s Blue Hole, named for a family who owns nearby land, is popular among divers. It is located on Long Island, Bahamas in a bay west of Clarence Town.

The sides of this blue hole are not entirely vertical. At the top, Dean’s Blue Hole is approximately 82-115 feet (25-35 meters). This diameter drops down vertically approximately 66 feet (20 meters) and then widens into a cavern that is approximately 330 feet (100 meters) across.

The Blue Hole Near Gozo, Malta
The Blue Hole Near Gozo, Malta | Source

The Blue Hole near Gozo, Malta

Malta’s Blue Hole is the island’s most popular dive site. This blue hole begins 23 feet (7 meters) below the water’s surface.

Inside the blue hole, 72 feet (22 meters) below the water’s surface, a cave branches off from the hole. By swimming through a short passageway, or chimney, divers enter an additional area within the blue hole containing coral gardens teeming with life.

The video below takes a while to buffer and load, but is well worth the wait. The colors of the soft corals, the calcium deposits, the algae and the cyanobacteria all mixed together are quite stunning. The various fish swimming and grazing on the cave walls along with the polychaete worms, also known as bristle worms, that crawl along the walls at the middle of the video are also a sight to behold. Journey even deeper with the diver and explore dimly lit caverns.

This video of the Blue Hole in Malta also takes a while to buffer. This video shows different sea life than the previous, including a jellyfish, tube worms, and a starfish.

Blue Hole and Canyon in the Red Sea, Egypt
Blue Hole and Canyon in the Red Sea, Egypt | Source

The Blue Hole Near Dahab, Egypt in the Red Sea

The Blue Hole in the Red Sea, just north of Dahab, is known as the “World’s Most Dangerous Dive Site” and has even been nicknamed the "Diver's Cemetery". This blue hole is approximately 427 feet (130 meters) deep. Although the approved recreational diving limit is 131 feet (40 meters), a tunnel located 170 feet (52 meters) tempts many divers to go deeper, some to their death. The entrance to this passageway is difficult to find making it even more dangerous for those who are tempted to go deeper than the 131 feet (40 meter) limit to explore this tempting tunnel. This blue hole and the surrounding area offers an abundance of coral and fish for a diver’s pleasure.

Swimming Through the Arch

The Blue Cave Korcula, Croatia

Croatia’s favorite dive spot, the Blue Cave (Modra Spilja), is not very deep – only 115 feet (35 meters). This minimal depth makes visibility inside the blue hole much clearer. Various types of marine life lining the walls is easily viewed and enjoyed.

Click here to watch a great video of this blue cave.

Watch the video below as a tour group enters and tours the caves.

Above the Water in the Blue Cave Korcula

El Zacatón sinkhole, once considered to be bottomless, is the world's deepest sinkhole.
El Zacatón sinkhole, once considered to be bottomless, is the world's deepest sinkhole. | Source

The El Zacatón Sinkhole

The El Zacatón sinkhole is the Earth’s deepest water-filled sinkhole, and it is also a blue hole. It’s roughly circular opening is approximately 360 feet (110 meters) across. But more amazing, it is 1099 feet (335 meters) deep! That’s more than 3-1/2 football fields deep!

It is from the floating islands of grass (zacate) that El Zacatón received its name. These islands of grass driven by the wind blow across El Zacatón regularly. Some of these islands of grass are quite large.

Until 2007, the pit was considered to be bottomless, and divers have lost their lives trying to find its bottom. Its depth was finally resolved through the use of NASA’s DEPTHX (Deep Phreatic Thermal Explorer) project’s robot.

Other Blue Holes

There are many other blue holes located around the world.

Another Blue Hole (Castalia) of interest, located in Castalia, Ohio, is approximately 75 feet (23 meters) in diameter and approximately 45 feet (14 meters) deep. Although it is filled with entirely freshwater (a spring fed basin), the waters are anoxic and will not sustain fish. This particular blue hole, which had previously been a tourist attraction, is now off limits to the public, but other sinkhole induced blue holes can also be found in the area.

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Copyright © 2012 Cindy Murdoch (homesteadbound)

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