Sitting on a bed of magma

 Herculaneum

Less than two years after Mount Vesuvius erupted, the seafront of the city of Herculaneum sank into the Bay of Naples.

The view of the Catholic church is that the city sank because it had become so debauched the Lord took out his revenge and drowned the city. 

A more likely scenario is that the eruption of Vesuvius destabilized the magma chambers under the city, and they in turn erupted destabilizing the ground around the seafront area which was already strained by the amount of building put on weak foundations.

The modern city stands on the same chambers as the old city, the ground is so unstable it is possible, with ease, to watch the ground move below your feet as the magma changes position. If these chambers rupture, the ensuing firestorm will make Pompeii appear like pricking a boil, as the city will be turned to ashes.