The Great Blue Hole: A 1,000ft wide "eye" into the Earth's ancient past

Located off the coast of Belize, the Great Blue Hole is the world's largest marine sinkhole. It’s a near-perfect circle, measuring over 1,000 feet across and 400 feet deep. Below the 300ft mark, the water becomes completely anoxic (no oxygen) and is filled with a thick layer of toxic hydrogen sulfide, creating a literal "graveyard" on the seafloor where anything that falls in is perfectly preserved in the darkness.

Source: Ian Bottle