It would be a rock climber's dream, were it accessible. But this 1600‐meter cliff is more likely to be descended than ascended. It is hundreds of meters below the sea.Two hundred kilometers off the Gulf Coast of Florida, along the edge of the continental shelf, nature has built a mile‐high precipice by piling up the refuse of millions of years of marine life. The carbonate‐rich shells of sea creatures resist erosion, but every so often enough cool fresh water seeps from the low‐lying Florida mainland to the lower face of the slope and dissolves the carbonate. As the sea floor weakens, it collapses violently, creating sheer cliffs as steep as 45°.