The eruption of Mount Vesuvius 2,000 years ago turned a victim’s brain tissue into glass. Scientists say they have figured out how this unusual phenomenon occurred.

It was a surprising discovery when scientists examining the remains of a man who died in bed in the ancient city of Herculaneum after Italy's Mount Vesuvius erupted in 79 AD found dark fragments resembling obsidian inside his skull.

A fast-moving current of hot gas and volcanic matter, also called a pyroclastic flow, followed, burying the area. Experts believe the ash cloud turned the man's brain into glass because the pyroclastic flow would not have reached high enough temperatures or cooled quickly enough.