http://en.wikipedia.org/wiki/Blue_Hole_(Red_Sea)
Blue Holes are intriguing dives, this one more so than most. It is a blue hole along the shore in Dahab, Egypt. This area is characterized by deep blue clear waters, abundant karst terrain and a blue hole with a 80 ft. long cave connecting the sink to the ocean. The connecting cave or arch is situated at 198 ft. however. There is a shallower spillway or "saddle" interconnecting the Red Sea to the blue hole at the surface.
An clip of two diver free diving down to, through the arch and up. That works out to about 475 ft. of swimming, including the roundtrip to 198 ft.!
A short description of the arch complete with wire frame 3 D graphics
I recall that Martin Stepanek hung out for photographers for about 30 seconds in the arch and on returning from a far deeper descent. I am having trouble finding the video though. Any help?