The sourced background on Blue hole.

From the air it looks like a pupil: a perfect circle of deep, dark blue staring up out of the turquoise shallows off Belize. The Great Blue Hole is nearly 300 meters across and about 125 meters deep — a marine sinkhole so sheer and so round that it seems artificial. It isn’t. It’s a drowned cave from the Ice Age, and its depths hold proof that the ocean was once a hundred meters lower than it is today.

The story starts when sea level was far lower. During the last ice age, so much of Earth’s water was locked up in glaciers that the ocean surface sat roughly 100 to 120 meters below its present level. Where the Blue Hole is now, there was dry land — a limestone island, part of the Belize Barrier Reef system, exposed to rain. And rain does to limestone what it always does: it dissolves it. Over tens of thousands of years, acidic rainwater carved a vast cave system into the island’s limestone, decorating it with stalactites and stalagmites the way caves everywhere are decorated.

Then the ice melted. The sea rose — slowly at first, then in pulses — and the island drowned. The cave flooded, its entrance collapsed, and the limestone roof fell in, leaving the circular shaft we see today. The drowning preserved everything: the cave’s passages, its columns, its stalactites, all intact beneath the sea. And those stalactites are the proof of the whole story, because stalactites can only form in air. Finding them 100 meters underwater means that rock was once 100 meters above the sea. The Blue Hole is a tide gauge written in stone, recording exactly how far the ocean has risen since the ice age.

It took Jacques Cousteau to make the world look. In 1971, Cousteau brought the Calypso to Belize and descended into the Hole, declaring it one of the great diving sites on Earth. His expedition confirmed the shaft’s near-perfect circularity and its depth, and the footage fixed the Blue Hole in the public imagination as the archetype of the mysterious deep. (A “blue hole” is now the general term for these drowned sinkholes; others include Dean’s Blue Hole in the Bahamas, one of the deepest known.) In 2018, a submersible expedition mapped the Hole in 3D and found, among other things, a layer of hydrogen sulfide at depth — a toxic, anoxic zone where nothing lives, the drowned cave’s stagnant breath.

Divers who descend into the Blue Hole pass through the layers of its history on the way down: the sunlit reef at the rim, the shadowed shaft walls, and then, at depth, the stalactites — stone icicles that grew drip by drip in a cave full of air, twenty thousand years ago, before the sea came back. It’s one of the few places on Earth where you can swim through climate history: every meter of descent is a chapter of the ice age ending.

The Blue Hole is part of the Belize Barrier Reef Reserve System, a UNESCO World Heritage Site. It will be there long after us — still circular, still dark blue, still holding its stalactites in the deep. And it will still be telling anyone who looks closely the same fact it tells now: the ocean has not always been where it is, and stone remembers.