Herculaneum and Pompeii were both destroyed by the same eruption of Mount Vesuvius in 79 A.D. For archaeologists, however, it must seem that they were leveled by different volcanoes entirely.
Pompeii was smothered beneath a shallow blanket of volcanic pebbles (lapillae) and dust. It has been relatively easy to excavate, and today two-thirds of the ancient city is visible to modern visitors.
Herculaneum, on the other hand, was buried alive. The full force of the volcano struck the site in 12 avalanche-like surges of mud and detritus, which shoved the sea itself forward by several hundred yards. Eruptions in later centuries buried the city even deeper, and the thick crust solidified into rock. Today, Herculaneum and its ancient shoreline lie 70 feet below ground level, the equivalent of a four-story building.