Now satellite radar technology has revealed the source of the symptoms. A rush of molten rock swelled the magma chamber under the volcano by some 13 to 26 million cubic yards (10 to 20 million cubic meters)—about 15 times the volume of London's Olympic Stadium—between January 2011 and April 2012. The ballooning chamber even forced parts of the island's surface to rise upward and outward by 3 to 5.5 inches (8 to 14 centimeters).

(See volcano pictures.)

The volcano has been quiet for 60 years, and these recent events don't indicate an imminent eruption, said geologist Nomikou Paraskevi of the University of Athens.

"It is a reminder that Santorini is an active volcano. It's just that it is currently in a