In the past two days, Rotary Club of Cromwell members escorted 400 people through the Brewery Creek tunnel, which was built to prevent landslips.
The tours, which teach groups about the tunnels, are run every two years to coincide with the Warbirds Over Wanaka International Airshow.
Club volunteers took groups wearing hard hats and vests through the Cromwell Gorge tunnel, which is mainly concrete-lined but has hard rock showing at the end.
It is one of 13 tunnels drilledin the Cromwell Gorge in the mid-1990s, towards the end of the Clyde Dam project.
At that time 17 potential landslips were identified in the gorge above what would become Lake Dunstan.
The 18.5km of tunnels were built for geological assessment of the hillsides and to drain groundwater from within and below the landslips, so when the lake was filled, the cliffside would not collapse.
Club member Richard Broadhead said a large rockfall could cause a tsunami, or a blockage which could flood Cromwell within 48 hours.
Geotechnical instruments in the tunnels still monitor the hillsides to warn of movement in the rock.
‘‘If something bad was about to happen, they would know about it instantly.''
The tours taught locals about an important part of history in their region, he said.
‘‘Then they can tell their friends about it when they come to visit.''
The tunnel's gate is usually locked, but Contact Energy gave the club permission to open it for its event.
The club has run the tours for about 15 years, using the proceeds for projects it supports.
- by Jono Edwards
