Scientists discovered the extinct volcanoes by accident as they were mapping the seafloor.

It is thought that the volcanoes, which are located around 155 miles from Sydney, could be 50 million years old. Researchers were mapping the seabed when their sonar detected the volcanoes, writes Michael Casey for CBS News.

50 Million-Year-Old Volcanoes Found Off Australia

Researchers stumble across underwater extinct volcanoes

The cluster of extinct volcanoes are in fact calderas, which form due to the collapse of the land surrounding a volcano when it erupts. The largest of the four has a rim that is almost a mile in diameter and is 2,300 feet above the seabed.

Extinct volcanoes such as these offer a clue as to the internal workings of our planet, says volcano expert Richard Arculus at the Australian National University’s.

“They tell us part of the story of how New Zealand and Australia separated around 40-80 million years ago and they’ll now help scientists target future exploration of the sea floor to unlock the secrets of the Earth’s crust,” Arculus said. “They haven’t been found before now because the sonar on the previous Marine National Facility research vessel, Southern Surveyor, could only map the sea floor to (about 10,000 feet) which left half of Australia’s ocean territory out of reach.”

Successful mission for international team

The primary purpose of the research was not to find underwater volcanoes, but rather to find the nursery grounds of larval lobsters.

“To be honest, it came as a complete surprise and it was sheer serendipity,” said University of New South Wales researcher Moninya Roughan, who was on the research vessel along with a team of 28 scientists.

“There are these incredible parts of the ocean that we’ve never ever mapped before, never seen before,” she said. “This was the last night of the voyage and we were hunting for lobster larvae … and we had just set a track or a path … and we just happened to go over these new volcanoes … So, it was sheer luck we passed over them.”

Not only did the team find the lobster larvae and the volcanoes, they also spotted young fish far further from shore than ever before.

“It was astounding to find juvenile commercial fish species like bream and tailor (93 miles) offshore, as we had thought that once they were swept out to sea that was end of them,” said University of New South Wales marine biologist Iain Suthers. “But, in fact, these eddies are nursery grounds along the east coast of Australia.”