As well as appearing in Guetemala, sinkholes are also ocean-bound. Dean's Blue Hole is the second deepest underwater hole, at 663 feet. Guillaume Nery is a freediver, a water based equivalent of a basejumper. He jumps into underwater sinkholes with no breathing apparatus, other than his lungs. This is him jumping into Dean's Blue Hole.
(:flash http://www.youtube.com/v/uQITWbAaDx0&hl=en&fs=1&color1=0x006699&color2=0x54abd6 width=425 height=344:)

The video is heavily edited. He doesn't actually get to the bottom. But it's freaky. Awesomely freaky.