Mount Vesuvius. This is what Pompeii looks like in the modern day, but in AD79, a volcanic eruption covered the ancient city of Pompeii with ash, preserving some pretty gruesome depictions of citizens in their dying states (google image them). The same eruption also covered a neighbouring city, Herculaneum, with a mud flow, also devastating the city and its inhabitants. 

You can virtually build a volcano here - http://dsc.discovery.com/convergence/pompeii/interactive/interactive.html - and change the viscosity and gas settings, its quite fun, especially compared to revising!