Scientists have discovered an underwater volcano that once extended over 3,000 feet above sea level and is believed to have helped form the island of Oahu.

The volcano started erupting some 5 million years ago in the channel between what is now known as Oahu and Kauai, according to a study published this week.

"We thought we knew where all the Hawaiian volcanoes were, and here's one we didn't really know about," said John Sinton, a geologist at the University of Hawaii at Manoa and lead author of the study this week, according to Reuters.

The volcano, named Kaena, is now dormant according to researchers.

Oahu is home to the state's capital and largest city, Honolulu, and is known for its two mountain ranges: Koolau and Waianae.

Until discovering the volcano, geologists believed Oahu was formed by the two volcanoes that created these ridges, according to Reuters.

Now geologists believe Oahu once had a third volcano, which has since receded back beneath the surface of the Pacific.

Kaena was the first-born of the three, though Waianae surfaced earlier some 3.9 million years ago. Kaena broke the surface some 400,000 years later, and Koolau broke surface 500,000 years after that, according to the study.

"By 3 million years ago, all three were emergent," Sinton said. "These islands are always subsiding even when they're growing. Once any volcano stops erupting they continue to sink. We just don't know when Kaena went underwater."

Researchers are still examining the location for more clues, like lava formations that show signs of erosion, according to Reuters.

Kaena likely didn't rise more than 3,300 feet above sea level, according to Sinton.

"It explains a lot of things about Oahu," he added.

The discovery was made while geologists were investigating an area off the western tip of Oahu, according to Reuters.

Research was published this month in the Geological Society of America Bulletin.

See Now: OnePlus 6: How Different Will It Be From OnePlus 5?