9 Kilometers
Mount Vesuvius, a highly active stratovolcano, was only 9 kilometers from what is now Naples, and a couple of miles from Pompeii at the time. The locals of the city had no idea the mountain was even a volcano, unfortunately.
Major Eruption
The last major eruption of Mount Vesuvius came in 1800 BC, long before the city of Pompeii had even been founded. No one within the settlement could know how dangerous it was to build a city that close. In the previous explosion, several Bronze Age settlements were devastated.