Blue holes, deep depressions ﬁlled with water sometimes so intensely blue they appear bottomless, are one of the spectacular formations of karst topography: limestone bedrock sculpted by the dissolving effects of rainwater or groundwater. Though there are many North American examples, such as the Blue Hole in northwestern Ohio, most famous is the Blue Hole sixty miles off the coast of Belize, a haven for deep-water scuba divers. Located in the aquamarine shallows of Lighthouse Reef, this blue hole is a near-perfect circle more than a thousand feet across, its deep indigo water a result of its four-hundred-foot depth. Though now submerged, it formed on dry land, either as a cavern whose roof ﬁnally collapsed or as a sinkhole. Then melting glaciers at the end of the Ice Age ﬂooded continental margins along the Atlantic coast, turning this land formation into one in the sea. The eerily pure color of a blue hole may not always signal depth: A shallow blue hole in the New Jersey Pine Barrens sometimes called the Jersey Devil’s Bathtub owes its saturated hue to the purity of strongly upwelling spring water—undoubtedly also the force that created the formation.