They are not like anything you have ever seen before and experts are baffled about why these secret underwater tunnels have been built.
Located beneath the ancient sandstone of Georges Head is a network of tunnels concealed on the sea floor, easily accessible to divers.
A manmade tunnel which leads down to a hidden staircase and cavern has been revealed.
Historians believe this could date back to the 1870s, when explosives were laid in the harbour to guard against an enemy invasion.
Historian Peter Hobbins said: "As an enemy ship goes past, you push the button, connect the wires, and potentially blow it up. You create what you would call a killing zone these days."
However experts are still trying to work out what the tunnel was used for.
During WWII, an anti-submarine boom net was built between Georges Head and Green Point. Historians say this could have been where it connected to land.
There are other theories. A tall shaft at the end could have been for ventilation, or for a quick getaway.
This extraordinary piece of history is just a 15 minute boat trip from Mosman in Sydney.