How an unsupported hull can break its keel

How could a large void under a ship snap its keel, and what determines the size of the unsupported area needed?

The void must remove water support from a substantial keel span without swallowing the whole hull, forcing the ship's structure to carry concentrated bending loads.

A void beneath a ship must be wide enough to leave a substantial length of keel unsupported but not so broad that the entire hull simply drops into a lower level. The surrounding water normally distributes pressure along the hull and helps carry its weight. Remove that support across the right span and the hull girder must bear loads in a pattern for which it was not designed. Even if the structure does not open immediately to the sea, severe bending can break machinery, fittings, and internal systems and produce an operational kill.