South Abaco Blue Holes

Great Abaco is limestone island built on the Little Bahama Bank, and its interior pine forest is riddled with what local cave divers estimate at more than 40 blue holes and underwater cave systems, most still unexplored. Like the rest of the Bahamas, these formed as dry caves during past glacial low stands, when sea level sat roughly 100 to 120 metres below today's, and then flooded from below as the ice melted through the Holocene. Inland caves here run fresh water down to a halocline at about 15 to 17 metres, below which the passages are tidally influenced salt water connected to the open ocean through the island's porous rock.

Cave diver Brian Kakuk has explored Abaco's blue holes since the early 1990s and is the founder of the Bahamas Caves Research Foundation, whose decades-long lobbying led the Bahamian government to declare the roughly 33,000-acre South Abaco Blue Holes Conservation Area in 2015, upgraded to a Queen's Commonwealth Canopy forest in 2019. Kakuk's own outfit, Bahamas Underground, based in Marsh Harbour, remains the only operator guiding divers into the caves; local operators have agreed that all cave diving on Abaco is guide-only, with no more than one or two divers per guide.

The reason this small cluster of holes draws scientists rather than only divers is Sawmill Sink, where Kakuk's 2005 discovery of tortoise and crocodile remains extinct on Abaco led to a multi-year excavation funded by the Bahamas' Antiquities, Monuments and Museums Corporation. Below the sink's hydrogen-sulphide layer, oxygen-free water preserved bones and soft tissue for millennia; researchers eventually catalogued more than 40 crocodiles and a new tortoise species from the peat. A branch museum built to house the finds was destroyed by Hurricane Dorian in 2019, and Sawmill Sink remains a permit-controlled research site, not one any operator sells as a recreational trip.