Iconic, volcanic and very much active.
There can be no more iconic image of Naples than the sight of the brooding volcano, Vesuvius. Best known for its eruption in 79D, witnessed and recorded by Pliny the Younger, the volcano and its activities has totally transformed the landscape of the area, not least by totally burying the towns of Pompeii (in ash) and Herculaneum (in volcanic mud). The fertile slopes now produce lovely pomodorini (tiny tomatoes) as well as grapes for the famous Lachryma Christi (tears of Christ) wine.
For details on walking to the top of the volcano (4,203 feet), see www.parconazionaledelvesuvio.it/grancono. Vesuvius is also a UNESCO Biosphere Reserve with marked out footpaths. The best way to see the volcano and the park is to take the train to Ercolano and pick up an EAVBUS (leaving every 30 to 60 minutes and taking around 30 minutes) which drops you as far up as vehicles can go, then there's a short walk (around 30 minutes) to the crater.
There have been more recent eruptions… one in 1906 killed more than 100 people and the last major eruption was in 1944.