Santorini’s volcano last had a major eruption 3,600 years ago, which buried the island beneath metres of pumice.
Since then, it’s been very quiet.
But now scientists reveal that the chamber of molten rock beneath the volcano, which lies in the Aegean Sea, expanded by 10 to 20 million cubic metres – up to 15 times the size of London’s 80,000-seat Olympic Stadium - between January 2011 and April 2012.
The team, led by Oxford University scientists, discovered that the magma has caused the surface of the island to rise by eight to 14 centimetres during that period.
What’s more, in January 2011, a series of small earthquakes began beneath the islands.
Most were so small they could only be detected with sensitive seismometers but it was the first sign of activity beneath the volcano to be detected for 25 years.
Locals have, however, become aware that the volcano’s behaviour has changed.
Team member Michelle Parks, of Oxford University's Department of Earth Sciences, said: “During my field visits to Santorini in 2011, it became apparent that many of the locals were aware of a change in the behaviour of their volcano.
“The tour guides, who visit the volcano several times a day, would update me on changes in the amount of strong smelling gas being released from the summit, or changes in the colour of the water in some of the bays around the islands.
“On one particular day in April 2011, two guides told me they had felt an earthquake while they were on the volcano and that the motion of the ground had actually made them jump. Locals working in restaurants on the main island of Thera became aware of the increase in earthquake activity due to the vibration and clinking of glasses in their bars.”
The scientists’ findings came from satellite radar images and Global Positioning System receivers that can detect movements of the Earth's surface of just a few millimetres.
Dr Juliet Biggs of Bristol University, also part of the research team, said: “People were obviously aware that something was happening to the volcano, but it wasn't until we saw the changes in the GPS, and the uplift on the radar images that we really knew that molten rock was being injected at such a shallow level beneath the volcano.
“Many volcanologists study the rocks produced by old eruptions to understand what happened in the past, so it's exciting to use cutting-edge satellite technology to link that to what's going on in the volcanic plumbing system right now.”
The team calculate that the amount of molten rock that has arrived beneath Santorini in the past year is the equivalent of about 10-20 years growth of the volcano.
But this does not mean that an eruption is about to happen - in fact the rate of earthquake activity has dropped off in the past few months.
"All registered activity is that of the ‘normal’ dormant state of the volcano,” Georges Vougioukalakis, a volcanologist with the Institute of Geology and Mineral Exploration in Athens, told Bloomberg Businessweek.
A report of the research appears in this week's Nature Geoscience.
The biggest volcanic eruption of the past 10,000 years took place at Tambora, Indonesia, in 1815.
It released such massive amounts of aerosols and dust into the atmosphere that temperatures around the world dropped.
The eruption killed 12,000 people.
Santorini, which is part of Greece's Cyclades, used to be a single island. What’s left now is the result of the huge explosion thousands of years ago, which left a giant water-filled caldera, 400m deep, behind.
Over 10, 000 people currently live on the island.