The Great Blue Hole is a large underwater sinkhole off the coast of Belize.
It lies near the center of Lighthouse Reef, a small atoll 62 miles from the mainland and Belize City. The hole is circular in shape, over 984 ft across and 410 ft deep. It was formed as a limestone cave system during the last glacial period when sea levels were much lower. As the ocean began to rise again, the caves flooded, and the roof collapsed. Believed to be the world’s largest feature of its kind, the Great Blue Hole is part of thelarger Belize Barrier Reef Reserve System.

The deepest part of ocean located in the Pacific Ocean, the Mariana trench , near
the Mariana Islands in Japan. The deepest part of this ditch (near the island of Guam), with a depth of 11,030 meters - about two kilometers deeper than the height of Everest) was detected in 1951 by the British research vessel Challenger II , which gave its name to the point, " The Challenger Deep "

Krubera-Voronja Cave, Also known as the Cave of Kruber, Voronja... Located in Abkhazia, Russia, it is the deepest cave in the world with recent measurements extending to a total depth of 2191 meters.
It was the first cave to be explored to a depth of more than 2 km down. On August 5th 2007, an international expidition with 56 members went in and the interesting thing is, they said that the cave system could be deeper.
"The caving game is far from over. It won't be; not as long as deeper abysses call out to be explored" said Alexander Klimchouk, a renowned speleologist.