Italian scientists have identified a huge underwater volcano 40km (25 miles) off the southern coast of Sicily.
The base of the volcano - named after the Greek philosopher Empedocles - covers an area larger than Rome.
The volcano is higher than the Eiffel Tower in Paris, with one peak just seven metres below the sea's surface.
Empedocles is dormant and shows no sign of imminent eruption. Mount Etna, Europe's largest active volcano, lies 100km (60 miles) to its north.
The structure - which incorporates peaks previously thought to be separate volcanoes - has a base that measures 750 square km and stands 400m (1,300ft) high.
At various times in history, Empedocles has formed a small island. The first recorded eruptions occurred in the third century BC and the last in 1831.
Its emergence then put it at the centre of an international row over to whom the volcano actually belonged.
New survey equipment was used to confirm that what used to be considered a series of small underwater fissures are in fact part of a single massive volcano.
"People used to think that there were small centres of emission, distant from each other," Cesare Corselli, president of the National Inter-University Consortium for Marine Science, was quoted by Reuters news agency as saying.
"The hypothesis... is that this is a singular volcano that, alongside Etna as an example, can have a central eruption or a series of lateral eruptions," he said.
The volcano was named Empedocles after the Greek philosopher who hypothesised that all matter consisted of four elements - earth, air, fire and water.