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. Read More: National Geographic
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