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