The Great Blue Hole — the world’s largest blue hole — is a massive underwater sinkhole off of the coast of Belize. It lies near the center of Lighthouse Reef, a small atoll 60 miles (96 kilometers) east of the Belize mainland. The hole is near perfectly circular in shape, more than 1,000 feet (305 meters) across and 480 feet (146 meters) deep.
Lighthouse Reef Atoll Blue Hole
The Blue Hole is the result of repeated collapses of a limestone cave system formed during lower sea level stands in the course of the last ice age.
The outer edge is merely a few feet underwater at high tide. It’s 480 feet (146 meters) deep instead of the shallower 390 foot depth because the atoll is on a geological fault block that’s been subsiding into the basin through geologic time. It’s not an easy place to reach.
This incredible geographical phenomenon is one of the most astounding dive sites in the world, made famous by Jacques-Yves Cousteau who declared it one of the Top 4 scuba diving sites on Earth. In 1971, he brought his ship, the Calypso and his 1-man submarines to the hole to chart its depths and examine stalactites suspended from overhanging walls.
Contrary to rumors, Cousteau did not lose his son Philippe here — he died elsewhere in a helicopter accident. Neither did Cousteau randomly use explosives to destroy the patch reefs while navigating the Calypso in the Blue Hole. He did selectively remove — by limited blasting — a very small area to enable the Calypsoto reach the Blue Hole.
It has been likened by some to a journey into a prehistoric place in time. Great depth creates the deep blue indigo color that causes such structures to be known as ‘blue holes.’ Except for 2 narrow channels, coral surrounds the hole and breaks the surface in many areas at low tide.
Pederson’s cleaning shrimp are everywhere inhabiting the ringed and knobby anemones, and neon gobies advertise their cleaning services from the various coral heads. Angelfish, butterflyfish, hamnlets, and small groupers are also commonly seen. Elkhorn coral grows to the surface and purple seafans sweep at the calm surface waters, glittering their rich hues.
The walls are sheer from the surface until a depth of approximately 110 feet (44 meters) where you begin to encounter stalactite formations which actually angle back, allowing you to dive beneath monstrous overhangs. The water is motionless and visibility often approaches 200 feet (61 meters).
The deeper areas inside the Blue Hole don’t have the profusion of life associated with most drop-offs as a result of poorly circulating waters and little light.
Locals feel it should be one of the Seven Wonders of the World. It’s protected by the Belize Audubon Society and is a Belize National Monument. It became a World Heritage site in 1997
For millions of years the Blue Hole was a dry cave in which massive stalactites and stalagmites slowly formed. When the last ice age ended thousands of years ago, sea levels rose to cover the cave. When diving the Blue Hole, you swim under what is left of the old ceiling to view the remaining stalactites and stalagmites.
A major earthquake likely caused the cave ceiling to collapse forming the sinkhole, and the upheaval had the effect of tilting Lighthouse Reef to an angle of about 12 degrees. All along the walls of this former cavern are overhangs and ledges, housing pleistocene stalactites, stalagmites and columns.
Evidence for this are the shelves and ledges carved into the limestone by the sea, which run the complete interior circumference of the Blue Hole at various depths. The first of these ledges is found between 150 and 165 feet (45 to 50 meters) and best visited on the south side.
The base of the ledge is perfectly flat and cuts back into the rock some 15 to 20 feet (5 to 6 meters). This creates an ever-narrowing cavern until the roof reaches the floor right at the back. V-shaped ledges cut into solid limestone bear stalactites, stalagmites and columns which do not exist in the shallower waters of the Blue Hole.
The deeper one dives into the Blue Hole, the clearer the water and the more breathtaking the scenery, as the array of bizarre stalactites and limestone formations which mold its walls become more complex and intense.
On the western side at a depth of 230 feet (70 meters), there is an entrance through a narrow tunnel into a large cavern. In total darkness the stalactites, stalagmites and columns exist in an undisturbed world.
The floor is covered with very fine silt which billows into great clouds with the slightest movement from a passing diver. In the farthest corner, another narrow tunnel leads upwards into a 2nd cavern and another leads to a 3rd cavern where the skeletal remains of turtles which found their way in, never made their way out.
Some of the tunnels are thought to be linked right through to the mainland, though it has never been conclusively proven. The mainland also has many water-filled sinkholes which are connected to caves and tunnels.
2-foot long cores revealed outstanding sedimentary laminations during a study by Robert F. Dill and divers from the Cambrian Foundation in 1997. There is no oxygen near the bottom, and hydrogen sulfide prevents bottom dwellers from burrowing and disturbing the sediment.
Preliminary analyses of the short cores showed fluctuating pollen, spores, mercury, and arsenic levels ranging from 15 and 21 ppm (parts per million). Other events recorded in the short cores included hurricane or large storm layers. The storm layers are light-colored and beautifully laminated.
The temperature at a depth of 130 feet (40 meters) is about 76 degrees F (24 C) the year round.
Lighthouse Reef, an atoll approximately 25 miles long and 10 to 12 miles wide, has a typical enclosed lagoon. The depths in this lagoon vary from 5 to 25 feet (1.5 to 7.5 meters), where many scattered coral formations known as patch reefs are found within.
Near to The Blue Hole, the Half Moon Caye Natural Monument encompasses 10,000 acres of the atoll and 15 square miles of surrounding waters.
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