A scientist has found a way to use earthquakes to predict when volcanoes will erupt.
Swiss scientist Bernard Chouet fell in love with volcanoes when he witnessed spectacular fountains of lava spewing from Sicily's Mount Etna in 1969.
Now at the US Geological Survey, Chouet has devoted his career to finding a way to predict deadly volcanic eruptions. He is haunted by a disaster in South America that killed 25,000 people.
When Colombia's Nevado del Ruiz volcano erupted in 1985, it melted a glacier capping the mountain. Water and volcanic ash combined to produce devastating mudflows that wiped the entire town of Armero off the map.
By then Chouet had developed a theory that volcanic eruptions should be preceded by a type of earthquake he called a long period event.
Chouet believed that long period events were a sign that pressure was building up inside a volcano.
When he finally saw the earthquake records from Nevado del Ruiz, a year on from the disaster, he was horrified.
Chouet saw long-period events all over charts. For days before the eruption the volcano had been screaming "I'm about to explode" but no one had heard the warning.
In the early 1990s another Colombian volcano, Galeras, became restless. Long period events had again appeared on the charts - a clear sign of an impending eruption, according to Chouet.
But US volcanologist Stanley Williams was sceptical about Chouet's approach. Apart from the long period events the volcano was completely quiet.
So on 14th January 1993 Williams led a group of scientists into the crater of Galeras to measure gas emissions.
It was a tragic misjudgement. As they were preparing to leave the crater the volcano erupted, killing six of his colleagues and three tourists. Williams himself was severely injured.
In December 2000 Chouet was vindicated in dramatic fashion. For several years the mighty PopocatÚpetl on the outskirts of Mexico City had been gently steaming.
But then the long period events started - so many that they merged into a continuous tremor that could be felt in nearby villages.
Using Chouet's methods scientists at the National Centre for Prevention of Disasters in Mexico City predicted that there would be a large eruption in two days. The government evacuated tens of thousands of people.
Forty eight hours later, bang on time, the volcano erupted spectacularly. It was PopocatÚpetl's largest eruption for a thousand years and yet no one was hurt.
The Horizon film "Volcano Hell" will be broadcast on BBC2 on Thursday 28th August at 9.10pm BST.