An ancient extinct volcano has been discovered four km underwater in one of the least explored areas of the Pacific Ocean.

The one-km-high seamount was found about 300km from Jarvis Island, an uninhabited island located halfway between Hawaii and the Cook Islands.

Scientists believe the seamount – which used to be a volcano – is 100 million years old.

It was found during a seafloor mapping mission led by James Gardner, research professor in the UNH-NOAA Centre for Coastal and Ocean Mapping/Joint Hydrographic Centre, who was looking to provide a clearer image of the outer limits of the US continental shelf.

He said the seamount appeared "out of the blue" but that the team was able to map it in its entirety.

seamount pacific ocean
The extinct volcano is thought to be about 100 million years old University of New Hampshire Center for Coastal and Ocean Mapping/Joint Hydrographic Center

Because so little exploration has been done of the area of the Pacific in which the volcano was found, Gardner said he was not surprised by the discovery.

"These seamounts are very common, but we don't know about them because most of the places that we go out and map have never been mapped before," he said.

It was found using multibeam echosounder technology that creates a detailed image of the seafloor. Most seamounts of the size of the one found cannot be identified by normal satellite data: "Satellites just can't see these features and we can," he explained.

At present further details about the unnamed peak, which lies about four km underwater, are unknown. However, Gardner said it may become of interest in the future.

"It's probably 100 million years old and it might have something in it we may be interested in 100 years from now," he said.

The volcano lies within the US continental shelf, meaning the US has jurisdiction of the sediment and rock that make it.