A World War II era tunnel at Waterfall that had been packed with explosives was recently discovered by road workers.
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And Transport for NSW believes there could be as many as 40 more in the area in unknown locations.
Transport for NSW Sydney Maintenance Director David Fishburn said the tunnel towards the eastern end of McKell Avenue was found by workers on road maintenance duty in the Royal National Park.
The chamber - now home to a colony of microbats - is about seven metres long, 1.2 metres high and lined with sandstone blocks and supported by eroded timber supports.
Research undertaken by the department suggest the tunnels were dug to lay a booby trap in the event of a Japanese invasion during World War II.
"We understand the chamber was dug under the road so that explosives could be hidden within it," Mr Fishburn said.
"In the case of an invasion, the explosives would have been detonated and the road destroyed to hinder the progress of enemy troops."
Transport for NSW said there could be as many as 40 other such tunnels - known as adits - in unknown locations in the Illawarra.
A Transport for NSW spokesman said they were built under secrecy by the former Department of Main Roads.
"Full details of the work including exact locations weren't revealed at the time for national security reasons, but it's believed that the 'explosives tunnels' were decommissioned from late 1942, with explosives removed," the spokesman said.
"Stone walls were to be constructed across the openings, however it seems that either this was not done in this adit under McKell Avenue, or the wall has collapsed away without trace."
During World War II the Royal National Park was used by the military preparing for an attack by Japan, which had invaded South-East Asia in 1942.
The tunnels were only one line of defence - road blocks, tank traps, gun emplacements and runways were all built or planned in 1942.