Fragments of the recent past loom out of bay's shallow waters

SOME of them are planes that fell from the sky, some are the ''semi-trailers'' of shipping in the 1800s, one of them is considered an early forerunner to the Spirit of Tasmania. All of them, in varying states of decay, are lying on the bottom of Port Phillip Bay.

While literally thousands of people take to the bay on most days during summer to enjoy the present, many of them would be oblivious of the fascinating snapshots of the nation's past lying just below them on the sandy bottom of the bay.

It may be quite a shallow body of water, but Port Phillip Bay is home to an estimated 170-180 shipwrecks. Some date back as far as 1840, such as the vessel believed to be the first lost at or near Port Phillip Heads, the cutter Prince Albert and its cargo of wheat, maize and mail. But shipwrecks continued to occur in the waters of the bay throughout the 1900s.

Very early in the 20th century, in 1903, one of the world's then biggest petroleum carriers, the SS Petriana, was wrecked after it ran ashore in fog on the Point Nepean side of the heads.

Visitors to the peninsula hot spots of Sorrento and Portsea this summer will be grateful that the collision happened long ago, for much of its cargo, 1300 tonnes of oil, washed up on the nearby beaches. Fortunately, it was in cans.

''It's wreck occurred on the heads but a whole lot of its cargo got washed in and washed up on the beach at Sorrento and Portsea,'' says Heritage Victoria senior maritime archaeologist Peter Harvey. ''It's now got a reef named after it - Petriana Reef.''

Most of the wrecks occurred before 1940, the year which saw one of the bay's most devastating collisions. Late in the evening of November 20 HMAS Goorangai sank in less than a minute after a collision in the south channel with the troopship MV Duntroon.

The HMAS Goorangai, a Royal Australian Navy 223-tonne minesweeper, was hit as it crossed the mouth of Port Phillip Bay while bound for Portsea. All 24 people on board the Goorangai were killed.

Collisions between vessels are responsible for some of the most damaging incidents in the bay, but most of the wrecks are due to a collision with something else - land.

''We find that the majority of shipwrecks that occur, occur because they crash into something - and it's usually land in one form or another. Port Phillip Bay has some special conditions, it's a fairly restricted area to navigate in because there are only fairly narrow shipping channels from the heads to the port. So, collisions were fairly common,'' Mr Harvey says.

''[But] the majority of shipwrecks occur because of navigational mistakes - they run into the land. Either the land poking out of the sea or the real land, and that's for a variety of reasons; storms, navigational errors, bad luck, poor mapping in the earlier days.''

Most of the bay's shipwrecks are accessible to recreational divers, except for a number of significant ones covered by special protection zones, such as the City of Launceston, an early passenger steamer.

Stephen Fordyce, chairman of Scuba Divers Federation of Victoria, says ''there are still a lot of really interesting things to see'' in the bay away from the protection zones, despite the passage of time.

''Probably the most popular and best known is the Eliza Ramsden. It was a clipper ship. It hit a rock coming through the heads and didn't quite make it. It's a very good dive. Until very recently the clipper bow was intact and upright, which was pretty cool,'' he says.

''It has fallen over in the last couple of years, on its side, but it's still recognisable. It's got that really nice curve on it. The wreck still sticks up about eight metres off the bottom,'' he says.

The Eliza Ramsden sank in 1875.