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Of all the names given to geological formations, "blue holes" have perhaps the most straightforward. For marine researchers, however, they're anything but. The watery sinkholes hold treasure troves of information.
"Due to the sediment build-up over thousands of years, blue holes can be like time capsules for the ocean," said Australian marine biologist Johnny Gaskell.
Gaskell recently explored one such blue hole in a remote location in the Great Barrier Reef. There, he shot video to capture what's nearly impossible to see from above.
At its center, the blue hole is just shy of 100 feet deep. An abundance of marine life like turtles and tropical fish inhabit the waters.
"Within the walls of the blue hole it is perfectly still, no current at all," said Gaskell. "What stands out is the large coral colonies that have grown into interesting shaped formations, likely due to the lack of current and disturbance from wave action. These colonies form abstract structures that seem to have no pattern, growing outwards and changing course at random."
According to Gaskill, these coral growths extend only a portion of the way down. Once the divers descended to the blue hole's dark bottom, it was mostly sandy sediment.
Oases of Life
The high walls lining sinkholes tend to preserve what lies below from damaging weather. Large hurricanes can be particularly dangerous for corals, which are smashed by swells.
It was after Cyclone Debbie hit Australia late last March that Gaskell began searching for blue holes. He was searching for corals that were spared from the storm.
Looking on Google Maps, he spotted one in a remote location, nearly 120 miles away from the nearest island. While the specific blue hole had been previously identified, its remoteness made it difficult to access and little was known about it. Gaskell was able to finally confirm that it was a blue hole during a diving expedition in September, but is only now releasing footage of the dive with National Geographic.
What Makes a Blue Hole?
Most blue holes are formed from sinkholes or caves that develop slowly over time, as rock begins to erode and collapse. Many blue holes were formed during the last Ice Age, after sea levels rose and filled existing sinkholes with water. The term blue hole simply comes from the dark, navy waters that characterize the formations. Often, that water creates a striking contrast with the turquoise that surrounds them.
Gaskell plans to continue looking for and surveying blue holes in the Great Barrier Reef.
"Some of these sites have had scientists explore them in the past, but due to the remote offshore location, there are still parts of the Great Barrier Reef that remain a mystery," he said.