blue hole
marine sinkhole or cavern
Blue Hole
submarine sinkhole a few kilometres north of Dahab, Egypt
Great Blue Hole
submarine sinkhole off the coast of Belize
Dragon Hole
deep underwater sinkhole in the South China Sea
Amberjack Hole
Blue Hole 48 km off the coast of Sarasota, Florida
Dean's Blue Hole
world's deepest known blue hole
Blue Hole (Guam)
Submarine sinkhole off the west coast of Guam