Melbourne: Scientists claim to have discovered two huge sunken islands in the Indian Ocean, which once formed part of the last link between India and Australia.

A team from Sydney University, Macquarie University and the University of Tasmania say the two islands, about the size of Tasmania, were once part of the supercontinent Gondwana and are more than 1.5 kilometres underwater.

They say the islands, discovered west of Australia, were once above water. The discovery was made while the scientists were mapping the seafloor of the Perth Abyssal Plain.

"The data collected on the voyage could significantly change our understanding of the way in which India, Australia and Antarctica broke off from Gondwana," University of Sydney geologist Dr Joanne Whittaker, a team member, said.

The scientists returned to Perth last week after carrying out the complicated task of dredging up hundreds of kilogrammes of rock samples from the steep slopes of the two islands during a three-week expedition aboard a vessel.

Travelling on the Southern Surveyor, they discovered the islands through detailed seafloor mapping and by dredging rock samples from the steep slopes of the two islands, now in water depths of over 1.5 kilometres, the Australian media reported.

"We expected to see common oceanic rocks such as basalt in the dredge, but were surprised to see continental rocks such as granite, gneiss and sandstone containing fossils," Dr Simon Williams, the chief scientist on the expedition, said.

He added: "A detailed analysis of the rocks dredged up during the voyage will tell us about their age and how they fit into the Gondwana jigsaw."

The islands, called "micro continents", were formed when India began to move away from Australia, 130 million years ago during the Cretaceous  period, when dinosaurs roamed the Earth. They became stranded thousands of kilometres from either coast as the land masses separated, say the scientists.

"The sunken islands charted during the expedition have flat tops, which indicate they were once at sea level before being gradually submerged.

"Our preliminary analysis of the magnetic data that we collected could cause us to rethink the plate tectonic story for the whole of the Eastern Indian Ocean," Dr Whittaker said.

(Agencies)