Geologists have discovered a huge volcano, as big as Britain and Ireland combined.
Tamu Massif is part of the Shatsky Rise, a deep plateau on the floor of the Pacific Ocean, about 1600km east of Japan.
It has an immense lava dome which was formed from an eruption around 144 million years ago.
Until now, it had been thought to be a vast system of multiple volcanoes, but researchers from Texas A&M University discovered Tamu Massif is a single entity.
Their data included 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.
The find has been reported in the journal Nature Geoscience.
Team leader William Sager told AFP via email that it seemed 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.