Herculaneans, upwind of the ash column, may have thought themselves to be safe. Although shaken by the earthquakes that accompanied the eruption, the city, which was much closer to Vesuvius than was Pompeii, received no ash fallout. Around 1AM on August 25th though, the volcanic column collapsed, sending a wave of superheated gas and liquefied volcanic matter racing down the side of Vesuvius directly toward Herculaneum. Reaching speeds approaching 100 miles an hour and temperatures of approximately 750 degrees Fahrenheit, this wave, known as a pyroclastic surge, instantly killed everyone in its path. In all, six pyroclastic surges would strike the city of Herculaneum, covering areas to depths as great as 75 feet.

In Pompeii, many people died as they took shelter in buildings-- perhaps hoping the eruption would subside, or waiting for an opportunity to make a dash for safety. Already weakened by earth tremors, many buildings collapsed as ash and stone accumulated on the roofs, killing those inside. Some of the elderly, infirm, and the very young would have been overcome as they breathed air heavy with ash and sulfurous gas. In the early morning, many of those left alive in the city finally ventured out to attempt an escape through the city gates and out into the countryside. Around 7:30AM, the third pyroclastic surge struck. This surge, the first to reach Pompeii, killed all who remained alive. Three more surges struck, turning once-vibrant cities into cities of the dead.