Inside a blue hole in the Great Barrier Reef

[Still Waters]

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.

https://news.nationalgeographic.com/2017/12/underwater-blue-hole-explored-great-barrier-reef-video-spd/