The hydrographic survey clearly shows the Sydney Harbour Tunnel, as well as a 45m deep hole to the west of the bridge.

Sydney Ports has revealed some of its 3D imagery of the floor of Sydney Harbour, which it uses to maintain the shipping lanes and berths.

There are four hydrographic surveyors at Sydney Ports who monitor the bottom of the Harbour to make sure that obstacles do not disrupt water traffic on Port Jackson.

Venessa O’Connell, Andrew Tsaccounis told the Daily Telegraph that they and their colleagues use a $750,000 multi-beam echo sounder system 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 told the Daily Telegraph. “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.”

These objects have previously included: concrete blocks and steel poles for construction work; various items of furniture; shopping trolleys; small boats that have broken from their moorings and later sunk; and even cars and motorbikes.

The survey also shows the location of many 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.

You can read more, including more images and a video, over at the Daily Telegraph.