I'm presently in the middle of the Dr Who episode "Fires of Vulcan", which has inspired me to do some research:
The city of Pompeii was destroyed when Mt. Vesuvius erupted around 2000 years ago. Popular depictions of the disaster show people fleeing from huge banks of lava tumbling down the mountainside. That would have been terrifying - large lava flows can move in excess of 100 miles per hour - but it's not what actually happened to the city. It was instead buried up to the rooftops in blinding, suffocating drifts of poisonous hot ash. If ten feet of ash isn't enough to make you abandon your property, consider what happened to nearby Herculaneum: It was buried under seventy five feet of ash, rolling down the hillside in scalding hot waves, each flowing over the last.
For a long time, archeologists believed that the citizens of Herculaneum all escaped to safety - but in the 1980's, hundreds of victims were unearthed from the buried remains of a boathouse at the far end. Seems they all took refuge there, and eventually succumbed to poisonous gases as the pyroclastic flow enclosed their bodies.
When archeologists dug the bodies out, many of them were missing the tops of their skulls. Apparently, in the heat upwards of 750 degrees, their brains had exploded.
Okay, now, everybody have a nice day! :D