SPORTS cars, 100-year-old shipwrecks and mysterious 45m-deep holes are just some of the secrets hidden deep beneath Sydney Harbour.

The team responsible for keeping the Harbour’s busy shipping lanes and berths clear of obstacles have now revealed some of the spectacular 3D images of the Harbour floor and its surprising contents.

Four hydrographic surveyors at Sydney Ports keep an ever-vigilant eye on the bottom of the Harbour to make sure possible dangers in the depths do not disrupt water traffic on Port Jackson.

Venessa O’Connell and Andrew Tsaccounis and their colleagues use a “multibeam echo sounder’’ system, valued at $750,000 to make detailed 3-D maps of the Harbour floor.

Installed on-board the tri-hulled motor boat “Port Explorer”, the system fires 512 separate sonar rays to the bottom to build up a digital map of the Harbour’s hidden depths.

“We are looking for anything that can impact on shipping and shipping schedules,” Ms O’Connell said.

media_camera 3D splendour...Sydney Harbour as you’ve never seen it. Four hydrographic surveyors at Sydney Ports have charted the harbour with colours representing the different depths.

media_camera Deep secret.... A 45-metre deep hole, just west of the Harbour bridge.

“While Sydney Harbour doesn’t really have any problems with filling up with silt, we do find a wide variety of objects that have been dumped or fallen off the back of ships and barges.”

During their regular scanning “runs”, crisscrossing the Harbour’s shipping lanes, the surveyors have found massive concrete blocks and steel poles from construction work; a variety of furniture items included tables and chairs; dozens of shopping trolleys; small pleasure boats that have broken free from moorings and sunk and; cars and motorbikes.

Mr Tsaccounis said they found a brand new Toyota Landcruiser 4WD 50m from the wharf at White Bay.

“We still can’t figure out how it got that far out,” he said. “It may have fallen off the back of a ship.”

media_camera Fine detail....The Currajong Wreck off Bradley’s Head. Source: Supplied.

media_camera Mystery...Image of wrecks off Balls Head. Source: Supplied

media_camera Ghostly image... a sailing vessel in the deep. Source: Supplied.

The team recently spotted the distinctive outline of what looks like a Porsche or Audi sports car in 6.5m of water just a few metres from the edge of Pier One at Dawes Point, almost under the Harbour Bridge.

It is still there because it is not considered a hazard to shipping.

The survey also shows the location of historic items including the Harbour’s biggest and most intact wreck, the TSS Currajong, a collier that was sunk just off Bradleys Head, near Mosman, in 1910 after being hit by the SS Wyreema, a 6000-tonne passenger liner.

The Currajong is in a shipping lane, but lies in about 30m of water, one of the deepest parts of the Harbour.

“Thousands of people have passed over the Currajong, and have no idea it’s down there,” Ms O’Connell said.

“There is also a 45m deep hole in the Harbour floor just west of the Harbour Bridge, close to Blues Point, that most people have no idea exists.”

media_camera Echo scan of car off Pier 1, Dawes Point.

media_camera Wreck.. A Landcruiser is pulled from White Bay.

media_camera The wreck of a landcruiser is pulled from the Harbour.

Just off Balls Head, near Waverton, another 35m hole has a few little wrecks in and around it.

The giant holes come up as dark blue on the survey images.

The maps also pick up the two straight lines of the Sydney Harbour Tunnels, running just west of the Opera House to Kirribilli. Sydney Ports’ harbour master Philip Holliday said the work of the survey team is critical for the safe navigation of shipping not only in Sydney Harbour, but Port Botany, Yamba and Eden as well.

“The Harbour has a colourful history of debris and objects that have been found by our survey teams,” Captain Halliday said.

“We’ve also had to recover a steel pipe, about 6m long and 15cm across, that we found speared into the mud of Sydney Harbour’s main eastern channel with

its top only about 9m below the surface.”

“It’s all about ensuring our busy working ports remain safe for commercial and recreational traffic,” Captain Holliday said.

media_camera Hydographic surveyor Venessa O'Connell aboard Port Explorer, Hydrographic survey vessel. Picture Craig Greenhill