A perfectly circular hole in the ocean is really a collapsed cave

The Great Blue Hole sits in Belize's Lighthouse Reef Atoll, a near-perfect circle about 300 metres across and over 120 metres deep. It formed as a limestone cave system that collapsed in stages as sea levels rose after past ice ages — stalactites recovered from inside could only have formed in dry air, proving it once stood above water. Jacques Cousteau explored and filmed it in 1971, turning it into one of the world's most famous dive sites almost overnight.

— Wikipedia contributors, Great Blue Hole — Lighthouse Reef Atoll, Belize