Into the abyss

Cracroft's caverns

Ever wondered what lurks inside the Port Hills?

They are a forgotten part of Christchurch's military history, now being used to monitor the earth's rhythms. Wayne Martin goes underground, armed with his camera.   

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.

Recently, the caverns have been closed because of instability. Their reopening was announced by the removal of a single line of text deep in the council's website - easily overlooked, except I know where to look, and front up for a tour.

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. 

Near ground level, a stainless steel tube emerges from a timber-framed shed and darts left into a side tunnel. The science relies on two laser beams deflected by mirrors in opposite directions around the rectangular circuit. Any movement in the cavern floor results in one beam having to travel further than the other to complete the circuit. This causes a phase shift in the standing waves of the two beams, from which is calculated the rotation of the earth and even movement in the hills due to sea tides.

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.   

*Read more in the November issue*