Blue Hole, Natural sinkhole dive site in northern Agat Bay, Guam.
The Blue Hole is a natural sinkhole in Agat Bay, Guam, that drops vertically through the reef at several depth levels. The opening sits at around 60 feet (18 meters) below the surface and the formation continues downward through limestone and coral rock.
The sinkhole gained recognition as a dive site as modern diving equipment became more accessible in the latter half of the 20th century. Scientific surveys in the early 2000s identified new marine species living inside the limestone passages.
The Blue Hole is well known among divers on Guam and local dive schools regularly bring students here for training dives. The site draws people from many different countries, making it a meeting point for the local diving community.
Currents through the site can be strong, so divers should check conditions before getting in and always carry a surface marker buoy. A local guide helps with navigation through the passages and is a good idea for anyone not familiar with the site.
The sinkhole has a heart-shaped opening visible from the surface, which is easy to spot when looking straight down from a boat. At around 130 feet (40 meters) depth, a natural window opens directly onto the open ocean, giving divers a sudden and unexpected view of blue water beyond the reef.
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