Why Is There a Dark Circle in the Middle of the Reef? A Cave Roof Fell In

During the ice ages a great deal of the world's water was locked up in ice sheets and sea level stood well over a hundred metres lower than today, which left large areas of what is now shallow seabed exposed as dry limestone platform. Rain falling on that limestone is slightly acidic and dissolves it along cracks, creating cave systems exactly as it does in limestone country today, and where a cave roof became too thin it collapsed, leaving a steep-walled shaft open to the sky. When the ice melted and sea level rose, those shafts filled and the surrounding platform became shallow sea, leaving the deep holes surrounded by reef.

The contents are unusual and follow from the shape:

• •Steep or overhanging walls with almost no light below the rim
• •A layer of dense water below that does not mix with the sea above
• •No oxygen at depth, which excludes almost all animal life
• •Hydrogen sulphide, which is toxic and smells of rotten eggs
• •Stalactites and stalagmites, proving the cave was once dry
• •Undisturbed sediment on the floor, layer upon layer

The still water below the rim is the feature that makes these places scientifically valuable and physically dangerous. Fresh or brackish water from rainfall and from the rock floats on denser salt water, and the shaft shelters both from wind and current, so a sharp boundary forms and persists, sometimes several of them stacked. Below the boundary nothing brings oxygen down, and bacteria consume what little there is while producing hydrogen sulphide. Divers describe the boundary as a visible shimmering surface within the water, and crossing it means passing into water that is a different temperature, a different chemistry and frequently completely opaque, which has killed a considerable number of people.

Not all of these features are at sea, and the inland ones have a quite different cultural history. Where the same collapse happened away from the coast the result is a water-filled shaft in the land surface, and the best known concentration is in the Yucatan peninsula, where thousands of them provided the only reliable water in a landscape with almost no surface rivers. Maya settlement locations track them closely, and several were used for offerings, with excavation recovering pottery, jade, incense and human remains deposited deliberately over centuries. Similar inland shafts occur in Florida, the Bahamas, Australia and China, and the deepest known example anywhere is a marine one in the South China Sea at over three hundred metres.

An oxygen-free floor with no burrowing animals and no currents is close to a perfect archive, and cores taken from these holes have produced remarkable records. Layers accumulate undisturbed year after year, trapping pollen, charcoal, dust and the shells of surface-dwelling organisms, which gives a continuous local record of climate and vegetation stretching back thousands of years. Coarse layers deposited by hurricanes have been used to build hurricane histories for the Caribbean far longer than written records, showing that storm frequency varies over centuries. Human and animal remains have been found in several, including material that fell in when the shaft was dry, and the preservation is exceptional.

Rain dissolved caves in limestone exposed when ice age sea level stood over a hundred metres lower, roofs collapsed into shafts, and rising seas drowned them, which is why a deep circular hole sits in a shallow reef. Lighter water floats on denser water inside the sheltered shaft and stops mixing, so the depths hold no oxygen. The undisturbed floor sediment records hurricanes and climate for thousands of years.