They were looking at the Twelve Apostles formation off Great Ocean Road, Victoria, when one of the towers crumbled into rubble, destroying in seconds a 70m (230ft) limestone monolith that nature had taken millions of years to create. A huge crack had opened up on the seaward side.

Last night the remains of fallen pillar — the second of the group, looking northwest from the main viewing platform — lay in a forlorn pile only metres above sea level.

A 15-year-old boy, on holiday from Sydney with his parents, took two photographs, one at 9.18am, the other at 9.19am. One minute the sea stack was there. Then it was gone.

Natasha Johnson, a Parks Victoria duty ranger, arrived on the sea front about 20 minutes later. “The family was still absolutely stunned by what happened,” she said. “They just couldn’t believe it. The mother said she had noticed the base of the structure shimmer and shudder and then all of a sudden it collapsed like an old building being demolished.”

Bernie Joyce, from the earth sciences department of Melbourne University and a member of the heritage sub-committee of the Geological Society of Australia, said that 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.”

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

The Great Ocean Road generated A$1 billion (£425 million) in tourism revenue last year, with hundreds of thousands of people driving the scenic road to visit the Apostles. Despite their name, there have only ever been nine apostles. Now there are eight.

“It won’t be the same sort of photo any more,” Sally Ryan, a tour guide with Otway Discoveries, said as she stood on the blustery front near Port Campbell, at the western end of the Great Ocean Road.

Andrew Gleadow, Professor of Earth Sciences at the University of Melbourne, said that 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 that it was only a matter of time before the remaining Apostles crumbled into the sea.

“They certainly haven’t got millions of years left . . . I would think it would happen every thousand years or so. (The Australian)

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