Cracroft Caverns - Inside the secret WW2 tunnels in Christchurch

Cracroft Caverns, also known as the Cashmere Caverns, are a series of large chambers in the hill of the Cashmere suburb of Christchurch, New Zealand.
Constructed secretly during the Second World War in response to the Japanese threat, they were intended to house operational headquarters in the event of attack. The military commandeered the Cracroft Wilson estate, founded by Sir John Cracroft Wilson in 1854, for their Southern Group headquarters, and work on the tunnels began in 1942.
The largest cavern was 7 metres high, 10 metres wide and 30 metres long. Prestressed concrete was used to line the caverns. Officers based in the estate house would come down stairs into the tunnels, while others were to enter through the access tunnels. Construction came to a sudden halt in 1944 as the threat of invasion receded, even though work on the bunkers was nearly complete. The weekend before the military left the estate, fire broke out and the Cracroft Wilson House was burnt to the ground.
The entrances to the underground complex were sealed after the war, and the existence of the caverns was largely forgotten until 1987 when TVNZ reporter Jeff Field was told of the caverns during a chance conversation with a gardener at Cashmere Hospital. Using the archives at the Ministry of Defence, Field discovered references to the caverns in newspaper articles dating from January 1945. The investigation was assigned to another reporter, Bill Cockram, who contacted the owner of Cashmere House which has been rebuilt following the fire. With the permission of the owner, excavations were carried out and the caverns were entered for the first time since the end of the Second World War.
The caverns now became the home of a series of ring laser experiments set up by the University of Canterbury's physics department, taking advantage of the extremely stable temperature in the caves.
There is now an associated small public park behind the Princess Margaret Hospital.
Parts of the caverns have periodically been opened to the public in the past, but due to damage from the 2010 Canterbury earthquake and 2011 Christchurch earthquakes there is currently no access.
This from the Christchurch Press in 2009 :
Christchurch's most closely guarded secret during World War 2, the Cracroft Caverns, retain something of their cloak-and-dagger origins. On the first Sunday of every month, the Christchurch City Council runs tours of the military bunkers tunnelled deep into the Port Hills, but the walks are not actively promoted.
No signs point to the secluded car park behind Princess Margaret Hospital from which a park ranger leads explorers along a steep unmarked path to a nameless glade high above the city. No signs identify the steel door that seals the railway tunnel-like entrance to the bowels of Cracroft Hill. All that's missing, it seems, are the blindfolds.
The Japanese attack on Pearl Harbour late in 1941, followed rapidly by the fall of Singapore and the bombing of Darwin, sent New Zealand's war machine into overdrive. Plans for an underground command centre in the Port Hills were drawn up. Modelled on Winston Churchill's War Cabinet rooms beneath London, the combined army, navy, and air force headquarters were designed to house the thinkers on whom the final defence of the South Island would depend.
The park ranger presses a tiny remote controller, the door unlatches and we enter a curving rough-ribbed tunnel slung with electric lights. The air feels warm and thick at about 20°C and 90 per cent humidity.
"It's like Auckland without the Aucklanders," the ranger observes, safe in the company of our small Cantabrian group. Conditions in the caverns stay constant year round due to the geothermal mass of the volcanic bedrock, he says.
The entrance expands into a musty chamber the size and shape of a cathedral. Smooth concrete walls, believed to be the first use of pre-stressed construction in New Zealand, rise to about seven metres. Curved beams keyed into the top of the walls arch overhead for half the chamber's length. Preserved by the hill's natural air-conditioning system, the concrete could have been poured yesterday.
Wall displays retell the history of the caverns and their rediscovery in 1987. Publicity at the time alerted the University of Canterbury to the perfect environment for its ultra-sensitive ring-laser experiment. Installed in collaboration with a German firm, the apparatus is the largest of its type in the world.
Through a side tunnel of the U-shaped complex, we enter a second cavern, less complete than the first. Dim, silent and uncluttered, it has more of a time-capsule feel than its neighbour.
Concrete stairs climb steeply into the shadows on the far wall. This was once the entrance to a 100-metre underground passage to the cellar of the Southern Group's military headquarters in the Cashmere mansion commandeered from the Cracroft Wilson family.
Thanks to Steve Austin and Bob Lazar for the footage.