The limestone beneath the Banks has been accumulating since at least the Cretaceous period, roughly 145 million years ago, and possibly since the Jurassic. All of it was deposited in shallow water, which presents a geological puzzle: how does a shallow-water formation grow to a thickness of 4.5 kilometers? The answer is subsidence. The platform has been sinking under its own weight at a rate of approximately 3.6 centimeters per thousand years, creating new shallow space at the surface even as the column grows deeper below. The result is a structure that records over a hundred million years of tropical marine history in its layers, a library written in calcium carbonate.

During past ice ages, when sea levels dropped as much as 120 meters below their present mark, the Bahama Banks were dry land. The Bahamas we see today, scattered islands and cays covering a modest area of ocean, represent only a small fraction of their prehistoric extent. When exposed to the atmosphere, the limestone underwent chemical weathering that carved the caves, sinkholes, and blue holes that divers explore today. These karst formations are fossils of a different kind, not of creatures but of climate itself, recording the rhythmic advance and retreat of ice sheets thousands of miles to the north. When the ice melted and seas rose again, the platforms flooded, and the slow rain of carbonate sediment resumed.

The Banks are not a gradual slope into deep water. Their edges are startlingly steep. Where the Great Bahama Bank meets the Tongue of the Ocean, the seafloor drops from a few meters to over 2,000 meters in a near-vertical wall. This is one of the most dramatic bathymetric transitions in the Atlantic, a cliff face hidden beneath the waves that dwarfs any canyon on land. The Tongue of the Ocean itself is a deep trench carved into the platform, so deep and acoustically isolated that the U.S. Navy has used it for submarine testing. Pilots crossing the Banks can trace the boundary with their eyes: the pale, sun-flooded shallows end in a sharp dark line where the abyss begins, a geological border as visible at thirty thousand feet as it is from a boat.

The term "Bahama Banks" usually refers to the two largest platforms: the Great Bahama Bank surrounding Andros Island, and the Little Bahama Bank beneath Grand Bahama and Great Abaco. But the geological story extends far beyond the Bahamas themselves. The Cay Sal Bank, the Caicos Bank and Turks Bank of the Turks and Caicos Islands, the submerged Mouchoir Bank, and the Silver Bank and Navidad Bank north of the Dominican Republic are all part of the same carbonate system, collectively called the Bahamas platform. This single geological province stretches across hundreds of miles of ocean, connecting islands and nations that seem unrelated on a political map. Beneath the surface, the limestone binds them together. The Banks also hold the world's largest deposit of oolitic aragonite sand, tiny round grains formed by the precipitation of calcium carbonate in warm, agitated shallows, a resource found almost nowhere else at this scale.