Road workers have stumbled across an underground booby trap dug during World War II in national parkland south of Sydney.
The forgotten tunnel - discovered under a main road leading into the Royal National Park - was excavated in 1942 to help defend the coast in the event of a Japanese invasion.
At the time it was packed with explosives meant to be detonated to hinder the progress of enemy troops.
Transport for NSW spokesman David Fishburn said the explosives were long gone.
"I can assure the public there are no longer explosives in the tunnel, which is now home to a colony of regionally-significant microbats, known as the eastern horseshoe bat," he said.
There could be as many as 40 similar tunnels throughout the Illawarra region designed to destroy access routes, Mr Fishburn said.
New roadblocks, tank traps, gun emplacements and runways were also built or planned in 1942.
Beaches were protected by barbed wire in the Sutherland Shire while street signs and names were removed.
Land mines were laid, boats were taken off the water so they couldn't be used by advancing attackers and the national park was used as a training ground by local troops.