Perched on top of Santorini's sheer cliffs is a world-famous tourist industry worth millions. Underneath is the fizzing risk of an almighty explosion.

A huge ancient eruption created the dreamy Greek island, leaving a vast crater and a horse-shoe shaped rim.

Now scientists are investigating for the first time how dangerous the next big one could be.

BBC News spent a day on board the British royal research ship the Discovery as they searched for clues.

Just weeks before, nearly half of Santorini's 11,000 residents had fled for safety when the island shut down in a series of earthquakes.

It was a harsh reminder that under the idyllic white villages dotted with gyros restaurants, hot tubs in AirBnB rentals, and vineyards on rich volcanic soil, two tectonic plates grind in the Earth's crust.

Prof Isobel Yeo, an expert on highly dangerous submarine volcanoes with Britain's National Oceanography Centre, is leading the mission. Around two-thirds of the world's volcanoes are underwater, but they are hardly monitored.

"It's a bit like 'out of sight, out of mind' in terms of understanding their danger, compared to more famous ones like Vesuvius," she says on deck, as we watch two engineers winching a robot the size of a car off the ship's side.

This work, coming so soon after the earthquakes, will help scientists understand what type of seismic unrest could indicate a volcanic eruption is imminent.

Santorini's last eruption was in 1950, but as recently as 2012 there was a "period of unrest", says Isobel. Magma flowed into the volcanoes' chambers and the islands "swelled up".

"Underwater volcanoes are capable of really big, really destructive eruptions," she says.

"We are lulled into a sense of false security if you're used to small eruptions and the volcano acting safe. You assume the next will be the........

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