Sawmill Sink

An inland blue hole in Abaco's pine forest whose oxygen-free deep water preserved some of the most complete fossils in the West Indies, including extinct tortoises, Cuban crocodiles and early human remains. Underwater work there needs a special permit.

Sawmill Sink is a roughly circular inland blue hole in the pinelands of south-central Great Abaco. Its opening is up to about 15.5 m across, and below the surface a cone of fallen rock and sediment slopes down from about 9 m to about 34 m, with cave passage continuing south from the shaft.

The sink is famous for its fossils rather than as a recreational dive. Brian Kakuk found a tortoise skeleton there in 2004, and a team led by David Steadman of the Florida Museum of Natural History, working with the Bahamas Caves Research Foundation and the National Museum of The Bahamas, went on to recover complete skeletons of an extinct tortoise and of Cuban crocodiles, birds, bats, reptiles, plant remains and human bones. The 2007 paper in the Proceedings of the National Academy of Sciences described the deposit as exceptionally well preserved.

The preservation comes from the water's layering. A fresh-water layer sits on top, a murky mixing zone rich in hydrogen sulfide lies below it, and the deepest water is salty and has no dissolved oxygen, so the bacteria and fungi that normally break down bone cannot survive. National Geographic describes the hydrogen sulfide in these layers as poisonous, and the layer also cuts off all daylight.

Because of these discoveries, published accounts report that any underwater activity at Sawmill Sink needs a special permit. The sink lies inside the South Abaco Blue Holes Conservation Area, designated in 2015.

Sawmill Sink is a vertical collapse feature in the island's limestone, connected to a cave that extends more than 600 m to the south. A talus cone (a pile of fallen rock and sediment) occupies the shaft between about 9 m and 34 m depth.

The PNAS study describes three layers: fresh water from the surface to about 9.5 m; a halocline and mixed zone from about 9.5 m to 27 m containing hydrogen sulfide that blocks all light; and oxygen-free salt water below about 27 m. National Geographic reported a bacterial layer that colours the water at roughly 9 to 11 m.

The sink produced the first complete fossil skeletons of a tortoise and a crocodile found anywhere in the West Indies. Radiocarbon dates on bones from its peat deposit span roughly 4,200 to 1,000 years ago, and an older owl-roost deposit probably dates to the last ice age. The extinct tortoise was formally named Chelonoidis alburyorum in 2009, with Sawmill Sink as its type locality. Other finds include dozens of Cuban crocodiles, an extinct caracara, a flightless rail and human bones, the youngest dated specimen being a human tibia. Researchers used the record to argue that human arrival, not climate alone, drove many island extinctions.

One surface opening. The sink is in a restricted conservation area and underwater work requires a special permit; no public access point is published.