ONE of the famous Twelve Apostles collapsed into a heap of rubble yesterday, destroying in seconds a landmark nature had taken 20 million years to create.

The 70m-high limestone monolith off Victoria's Great Ocean Road crumbled shortly before 9.20am, in a relatively moderate swell, after a huge crack opened up on the seaward side.

Only eight Apostles now remain. Yesterday's collapsed pillar - the second of the group, looking northwest from the main viewing platform - was lying as a forlorn pile of rubble last night just metres above sea level.

"One minute the sea stack, which geologists say could have formed 6000 years ago, was there. Then it was gone, crumbled into the sea.

Bernie Joyce, from the earth science department of Melbourne University and a member of the heritage sub-committee of the Geological Society of Australia, said waves would have been chipping away at the sea stack by a few millimetres every year.

"Some of it is very soft - a combination of limestone, sandstone and fossils," Professor Joyce said. "It was always going to happen but rain or a severe wave attack could have brought it on."

Professor Joyce said the sea stack that collapsed was like a chimney sitting on a flat platform under the sea. The collapse would have been triggered by a vertical crack sending several tonnes of rubble swirling into the ocean.

A similar phenomenon occurred at nearby London Bridge, a popular tourist attraction in the Port Campbell area, about 10 years ago.

In that case, the arch of the bridge connecting the mainland collapsed, turning the formation into an island from which a couple had to be plucked by helicopter.

Despite their name, there have only ever been nine apostles. Now there are eight.

Professor Joyce said other Apostles were also likely to disappear over time. But for every one that fell down there were more being formed as the coastline retreated.

Andrew Gleadow, professor of Earth Sciences at the University of Melbourne, said the Apostles were formed after the tips of headlands were separated from the mainland, eroded by the sea. "A headland can be cut off and become a new little island ... sometimes the erosion begins as sea caves," he said.

Professor Gleadow said it was only a matter of time before the remaining eight Apostles crumbled into the sea. "They certainly haven't got millions of years left ... I would think it would happen every 1000 years or so.