<P> During the last surge, which was very dilute, one meter more of deposits fell over the region . </P> <P> The only surviving eyewitness account of the event consists of two letters by Pliny the Younger, who was 17 at the time of the eruption, to the historian, Tacitus . Observing the first volcanic activity from Misenum across the Bay of Naples from the volcano, approximately 35 kilometres (22 mi), the elder Pliny launched a rescue fleet and went himself to the rescue of a personal friend . His nephew declined to join the party . One of the nephew's letters relates what he could discover from witnesses of his uncle's experiences . In a second letter the younger Pliny details his own observations after the departure of his uncle . </P> <P> The two men saw an extraordinarily dense cloud rising rapidly above the mountain: </P> <P> I cannot give you a more exact description of its appearance than by comparing to a pine tree; for it shot up to a great height in the form of a tall trunk, which spread out at the top as though into branches . (...) Occasionally it was brighter, occasionally darker and spotted, as it was either more or less filled with earth and cinders . </P>

How long did it take the boiling mud to flow the 4 miles from herculaneum to pompeii