The Blue Hole of Dahab, Sinai, Egypt has reached legendary status.
Not least of which a large cavernous tunnel at 50m+ leading into the open sea, called the Arch. The blue light entering through the arch gives the place an otherworldly feel. So much so and coupled with the sometimes lax requirements of the local dive operators that divers neither qualified nor outfitted with the appropriate equipment attempt the dive, with often fatal results.
You’ll need to have Nitrox, advanced Nitrox, extended range and decompression technique certifications – or Trimix – if you’re “bent” to do the Blue Hole.