Geologists say they've uncovered the world's biggest volcano, which is approximately the same size as the British Isles, underneath the Pacific Ocean.

Dubbed 'Tamu Massif', the volcano is part of the Shatsky Rise, a deep plateau on the floor of the Pacific located around 1,600 kilometres east of Japan, they said.

It comprises a single, immense, rounded dome in the shape of a shield, formed of hardened lava from an eruption around 144 million years ago.

It covers around 310,000 square kilometres - the equivalent area of Britain and Ireland combined -- and slopes upwards to a height of around 3.5 kilometres above the sea floor.

"Tamu Massif is the largest known single, central volcano in the world," the team reported in the journal Nature Geoscience.

In area, "it is... approximately the same as the British Isles or Olympus Mons on Mars, which is considered the largest volcano in the Solar System."

It adds: "Although Olympus Mons seems to be a giant because it is more than 20 kilometres in height, its volume is only around 25 per cent larger."

Olympus Mons, in addition, has relatively shallow roots, whereas Tamu Massif delves some 30 kilometres into Earth's crust.

Ocean surveyors had until now surmised Tamu Massif to be a vast system of multiple volcanoes, a kind that exists in about a dozen locations around the planet.

The realisation that it was a single volcano of truly massive size only came to light when the team, led by William Sager at Texas A&M University, sought an overview.

They assembled data from rock samples, taken from an ocean-floor drilling project, and a chart of the seabed, provided by deep-penetration seismic scanners aboard a survey ship.

Put together, the findings suggest mega-volcanoes found in other parts of the Solar System have cousins on Earth, says the paper.

"The Earth variety is poorly understood because these monsters found a better place to hide - beneath the sea," it argues.

Mr Sager says it seems unlikely that Tamu Massif was still active.

"The bottom line is that we think that Tamu Massif was built in a short (geologically speaking) time of one to several million years and it has been extinct since," he said.

"One interesting angle is that there were lots of oceanic plateaus (that) erupted during the Cretaceous Period (145-65 million years ago) but we don't see them since. Scientists would like to know why."

Other volcanic leviathans could be lurking among the dozen or so large oceanic plateaux around the world, he thought.

"We don't have the data to see inside them and know their structure, but it would not surprise me to find out that there are more like Tamu out there," he said.

"Indeed, the biggest oceanic plateau is Ontong Java plateau, near the equator in the Pacific, east of the Solomon Islands. It is much bigger than Tamu - it's the size of France."

The name Tamu comes from Texas A&M University, where Mr Sager taught for 29 years before moving to the University of Houston this year, he explained.

AFP