"These corals are evidence of an extinct coral reef," says the voyage's Chief Scientist, CSIRO Marine and Atmospheric Research's Dr Ron Thresher.

"Our sampling came up with some very old fossil corals of the type we are now seeing live and forming extensive reefs at depths of 800-1300 metres. This suggests that the reef extended much deeper in the past, but how long ago or why it died out, we don't know yet," he says.

The composition of deep-sea corals is used to determine past ocean conditions, such as temperature, salinity and the mixing of surface and deep-water layers, over tens to hundreds of thousands of years.

Dr Thresher says over the coming year the samples will be examined to determine when these newly discovered reefs existed and if their extinction can be related to long-term climate patterns.

The findings will provide ancient climate data that contribute to models of regional and global climate change, based on historical circulation patterns in the Southern Ocean.

He says that at times the submersible vehicle - or Autonomous Benthic Explorer (ABE), on loan from the Woods Hole Oceanographic Institution (WHOI) - was pushed off course while exploring the extreme depths and, in two cases, had its forward progress stopped altogether. Such movements enabled researchers to identify previously unknown and unexpectedly strong, deep currents.

"The voyage was a success despite some of the roughest conditions ever experienced by the team, particularly in deploying the ABE," Dr Thresher says.

The voyage is part of a collaboration between CSIRO's Wealth from Oceans Flagship, WHOI in the US, the National Science Foundation in the US, the marine division of the Australian Department of Environment, Water, Heritage and the Arts and, the Marine National Facility.

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