The existence of the tunnel, previously only known to a few, emerged after Macquarie University (which has the lighthouse as its logo) worked on a 3D imaging project in partnership with the Sydney Harbour Federation Trust to mark the 200th anniversary, at the end of November, of the building of the lighthouse which replaced an earlier crumbling tower. More details about what has been discovered are revealed at the lighthouse, which can be climbed during Sydney Open on the weekend of November 3-4. But there are still many unanswered questions. Despite the use of radar, the exact route of the tunnel remains unclear. Also something of an unknown is it's precise purpose, although there are theories. The secret entrance to the tunnel below the Macquarie lighthouse. Credit:Macquarie University Michael Rampe, senior learning designer at Macquarie University, said the Federation Trust brought to its attention "something which most people in Sydney don't know about which is that there is an abandoned WWII emplacement halfway down the cliff.

"They don't know where the entrance is, they have lost the tunnel into it and they had a reference to it. So as part of our exploration we flew a drone quite close to the tunnel and created a 3D reconstruction from that. "We came in with the technology and reconstructed it for virtual reality so we could put people in front of it on the cliff and explore it and see it in a way that nobody can now. The VR experience will be available for the 200th anniversary but there will also be a sneak preview with VR goggles for anyone who goes to the Sydney Open lighthouse tour. "We don't think the tunnel led to the lighthouse. We did a bit of work with ground penetrating radar which so far is inconclusive. We are looking for that tunnel but if you look at the plans for the lighthouse there's no hidden basement and it's on solid rock. It could have gone to the gatehouse, which has since gone, or to one of the properties next door." Loading Replay Replay video Play video Play video Sean Brawley, Pro Vice-Chancellor (Programs and Pathways) and Professor of History at Macquarie, said he didn't think the entrance looked like a traditional gun emplacement.

"If there was a gun there it would have been a relatively small calibre gun but it looks more likely to be an observation position or maybe you could have had a searchlight there," he said. "At the exit point is an overhang which looks like all-weather protection which you wouldn't associate with a gun emplacement." The Macquarie lighthouse is worth a visit in its own right. It has been repainted ahead of Sydney Open and that includes each of the 100 metal stairs (there's are small chair for a rest after each 25). Inside the lantern room of Macquarie lighthouse. Credit:Wolter Peeters Sadly there's no lighthouseperson to reminisce about past storms. The original clockwork, which had to be wound at regular intervals through the night, has been replaced by electric motors. The spectacular view is, naturally, 360 degrees.

Tickets for Sydney Open on November 3-4 go on sale today which, at 21 years old, gives the key to the door to see inside some of Sydney's most remarkable buildings. Old and new are represented among 60 sites. Mark Goggin, executive director of Sydney Living Museums said: "Since the first event in 1997, we have welcomed more than 76,000 people, both Sydneysiders and visitors, who have enjoyed exploring some 400 architectural gems. For anyone curious about Sydney, this weekend-long celebration of architecture is not to be missed." New for the itinerary program in 2018 are the Primus Hotel in Pitt Street with its art deco glamour, formerly the HQ of the Sydney Water Board; Carthona House (1894) a large Gothic revival private residence at Darling Point; new buildings The Beehive office and private residence Platform House, both DEZEEN nominated for best emerging architects globally and the Chief Secretary's office with memorabilia of Sir Henry Parkes, the 'Father of Federation'.