The seas have risen before and, in areas that border Port Phillip Bay, there is evidence that at the height of a phase of global warming about 5000 years ago, sea water was lapping what is now dry land.

The Holocene warming, which cut Tasmania from the mainland, drove seas inland by 10 metres to 15 metres every 1000 years.

The maximum, a phase of higher rainfall (20 per cent to 50 per cent greater than present) and increased temperatures (0.3 of a degree to three degrees higher), saw the water intrude 2.6 metres beyond the present bay shoreline.

There were people walking around in this very different landscape that can be mapped by shell beds and fossil finds.

In Williamstown, there are shell beds 2.9 metres above the contemporary shoreline.

At Essendon, shark and dolphin fossils have been dug up. Much of low-lying Geelong and parts of the Mornington Peninsula were under water.

Sea shells have been found in what was the Tootgarook swamp.

The Moonee Ponds Creek and the Yarra and Maribrynong rivers entered the bay as separate entities and not in the "engrafted" delta trench that they share today.

The Yarra mixed with sea water at Burnley. Waves washed just below Spencer Street.

Port Melbourne was under and Emerald Hill, the site of South Melbourne Town Hall, was an island.

Elwood was the shallows because the shoreline followed Brighton Road, Elsternwick.

The arc from Mordialloc to Olivers Hill in Frankston was under water, along the line of the Mornington Peninsula Freeway.

The Boon Wurrung Aborigines explained it as the punitive work of Bunjil, the creator spirit, who was angry with them for their bad behaviour.

They were forced to adapt with a new toolkit of smaller, multi-component stone tools and a commitment to cease their wickedness.

By 3000 years ago, for a more scientific reason, the sea began to retreat because of a drier climate.

Left behind was Melbourne's present terrain: the deep sands of the Yarra Delta and what was once the 15-kilometre-long Carrum Carrum "boomerang shaped" swamp.

The Edithvale-Seaford wetlands mark out the high water mark of a Holocene sea that became entrapped by sand drifts and slowly turned into a freshwater wetland.

Its environmental area is on Edithvale Road, Edithvale. Melway 93 D6.