Just south of Naples, lies the somma-stratovolcano Vesuvius. Up until 79 AD, it had been relatively quiet for hundreds of years and seemed dormant. What the Roman people occupying the towns around Pompeii did not know was that a series of earthquakes had hinted that the volcanic plug which sealed it was lifting and about to blow. And blow it did. At around 1pm on 24 August, Vesuvius began spewing volcanic ash and stone thousands of meters into the sky. When it reached the tropopause (the boundary between the troposphere and the stratosphere), the top of the cloud flattened, prompting the Roman Pliny the Younger to describe it to Tacitus as the shape of a Stone Pine tree. The prevailing winds at the time blew toward the southeast, causing the volcanic material to fall primarily on the city of Pompeii and the surrounding area. Since Herculaneum lay to the west of Vesuvius, it was only mildly affected by the first phase of the eruption. While roofs in Pompeii collapsed under the weight of falling debris, only a few centimetres of ash fell on Herculaneum, causing little damage but nonetheless prompting most inhabitants to flee.
During the following night, the eruptive column which had risen into the stratosphere collapsed onto Vesuvius and its flanks. The first pyroclastic surge, formed by a mixture of ash and hot gases, billowed through the mostly evacuated town of Herculaneum at 160 km/h (100 mph). A succession of six searing hot mudflows and surges buried the city's buildings, causing little damage and preserving structures, objects and victims almost intact.
The people of Herculaeum first ran to the sea to try and escape. But as the pyroclastic mudflow rapidly overran the town, the people retreated to the boat houses along the shore. After the eruption of Mount Vesuvius in 79 AD, the town of Herculaneum was buried under approximately 20 metres (50–60 feet) of ash, mud, and rock. It lay hidden and largely intact until discoveries from wells and underground tunnels became gradually more widely known, and notably following the Prince d'Elbeuf's explorations in the early 18th century. Excavations continued sporadically up to the present and today many streets and buildings are visible, although over 75% of the town remains buried under the modern town of Herculaneum.