A team of researchers aboard an ocean-going research vessel called RV Investigator has discovered a cluster of 50-million-year-old volcanoes off the coast of Sydney, Australia.

The cluster of four extinct volcanoes is 12.4 miles (20 km) long and 3.7 miles (6 km) wide and the seafloor is 16,043 feet (4,890 m) deep, with the highest point in the cluster rising up to 13,117 feet (3,998 m).

The center of the cluster is 33 31 S, 153 52 E, which is 154 miles (248 km) from Sydney Heads.

The volcanoes in the cluster are calderas, which form after a volcano erupts and the land around them collapses, forming a crater.

The largest is 4,920 feet (1,500 m) across the rim and it rises 2,300 feet (700 m) from the sea floor.

“These particular types of volcanoes are really important to geoscientists because they are like windows into the seafloor,” said team member Prof Richard Arculus of the Australian National University.

“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.”

The extinct volcanoes were discovered during a search for nursery grounds for larval lobsters.

At the same time RV Investigator was also routinely mapping the seafloor.

“On board RV Investigator we have sonar that can map the sea floor to any depth, so all of Australia’s vast ocean territory is now within reach, and that is enormously exciting,” Prof Arculus said.

“The 308-feet (94 m) Investigator has other capabilities that marine scientists in Australia have never had before, and the vessel will be key to unlocking the secrets of the oceans around our continent and beyond,” added team member Prof Iain Suthers from the University of New South Wales.