♪ PROF. CHRIS JACKSON: It's hard to make decisions during volcanic eruptions, whether to stay and guard your property and stay with your loved ones... or whether to flee.

NARRATOR: Volcanologist Professor Chris Jackson believes the Pompeiians would have had to act quickly to stand any chance of survival.

PROF. CHRIS JACKSON: Within the first hour to hour and a half of the eruption, the column grew from around 14 kilometers high to about 22 to 23 kilometers high.

That's about twice the altitude at which a commercial aircraft flies.

VOLCANO EXPLODES PROF. CHRIS JACKSON: The rising column, driving everything up skyward, would have caused a zone of low pressure, meaning that the air from the surrounding region was sucked in.

The weather was changed by this volcanic eruption.

The wind would have picked up as the wind swept along these narrow streets towards Vesuvius.

For people fleeing the city, racing down these streets, behind them would have been chaos, ash falling out of the sky.

We'd have seen lightning as electrically-charged particles started to interact.

It would have been almost like the end of the world.

But at this moment, people in Pompeii would still have had a chance to escape.

They'd have been disorientated, they'd have been scared, but they would still have had a chance to exit the city through one of the gates.

But any window that was open for the residents to leave... was rapidly slamming shut.