twelve apostles australia
Australia's Twelve ApostlesCookaa /CC

A set of 'Drowned Apostles' have been discovered submerged off Australia's south coast, not far from the iconic Twelve Apostles – ancient limestone stacks formed through erosion over millions of years.

Scientists found five additional stacks 6km offshore, 50m below the surface of the ocean. The discovery was made by University of Melbourne PhD student Rhiannon Bezore, who was analysing sonar data from a project surveying potential reef habitats for seal-life.

While looking at the sonar images, she noticed five unusual features located next to a submerged ancient coastal cliff. Further analysis showed they were the same as the limestone stacks that exist on the surface.

Drowned apostles
A geographic image of the Drowned Apostles - the tallest standing nearly 7m - with the submerged cliff face behind them.Rhiannon Bezore.

There are currently eight of the original Twelve Apostles left. They used to form part of the cliffs but over time the soft limestone was eroded, forming caves that eventually became arches. When the arches collapsed, stacks reaching up to 45m in height remained.

The Drowned Apostles – named so because of their watery home – were formed in the same way, but were submerged by rising sea levels around 20,000 years ago, at the end of the last Ice Age. This is the first time such stacks have been found preserved beneath the surface of the ocean. Normally, they should have been completely eroded before disappearing below the sea.

David Kennedy, a coastal geographer who supervised Bezore's discovery, said: "We are calling them the Drowned Apostles because if you had stood on this ancient cliff face over 20,000 years ago they would have looked largely the same as the current Twelve Apostles."

He said the speed at which sea levels rose meant there was not enough time for them to be eroded down to nothing – rising at a rate of between 10 and 20cm per year: "Sea levels probably rose at the end of the last ice age so quickly that the sea just ran across the top of these things without knocking them over. It is amazing that they survived. When we go into an ice age it is a very slow freezing process, but when it decides to melt it generally does it quite quickly."

A report on the discovery has been published in the Journal of Coastal Research. As the sea level rose, the height of the stacks was worn down – they stand at around 7m tall. Bezore said when she first saw them she thought they must have something to do with the Twelve Apostles because of their proximity and shape.

"As the drowned Apostles are found in the same geological setting as the current Twelve Apostles, it is reasonable to assume that they were formed under the same geomorphic processes, some 60,000 years apart," researchers wrote. "Were it not for the relatively quick submergence of the stacks, they likely would have continued to erode at a similar rate as seen with the modern sea stacks until they collapsed."