The Cashmere Cavern (also known as the Cracroft Caverns, or casually 'the cave') house the , and ring lasers. It consists of two parallel caverns connected at the west end forming a U shape. Each cavern is approximately 40 m long, 10 m wide and 7 m high. Two tunnels through the volcanic basalt bedrock connect the caverns at the east end and near the west end.

This cavern environment has unusually good mechanical and thermal stability (10 mK over a few hours). As such it has the potential to meet the ultimate performance limitations of large ring lasers.

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The Cashmere Cavern is located 30 m beneath banks peninsular in the residential Christchurch suburb of Cashmere. From a geological point of view this puts us inside an extinct volcano, essentially decoupled from the south island and southern alps.


Top secret World War 2 bunkers -- leaked to the media and then suppressed -- romance, aristocracy, fire, mystery novels and state of the art laser technology. It could be a plot of the next James Bond movie but amazingly enough all this is contained in the history of the little known Cracroft Caverns, situated in the quiet Christchurch suburb of Cashmere.

In 1942 the fate of the Second World War hung in the balance. New Zealand watched nervously as the Japanese empire spread throughout the South Pacific. A Japanese invasion of New Zealand seemed a very real possibility.

A part of the planned defence of the South Island was the military's formation of Southern Group, a combined Army, Air Force and Navy initiative designed to fight off invasion. Southern Group headquarters were established in 1942 when the armed forces commandeered the Cracroft Wilson homestead and estate on the Cashmere Hills.

The Cracroft estate was originally founded by Sir John Cracroft Wilson in 1854. Sir John was a holder of the Star of India and both he and his descendants have been prominent figures in Christchurch society. Gordon Ogilvie's masterly "The Port Hills of Christchurch." (Wellington: Read, 1978) gives an account of this outstanding pioneer, and amongst other things, Ogilvie has good photographs of the old mansion, a magnificent colonial building with beautiful native timbered staircases etc. and designed by renowned architect Samuel Hurst Seager.

On the surface there was nothing spectacular about the Southern Group headquarters at Cracroft, they looked like a typical military base with a Mess Hall, Airman's Quarters, etc. Underneath the ground however there was something out of the ordinary, a top secret set of caverns and tunnels designed to be the underground nerve centre for the military defence of the South Island.

The job of blasting the caverns into the hillside started in 1942. The work was undertaken by the Public Works Department under instructions from the military. The whole operation was conducted under the strictest of secrecy and constant vigilance of the military centuries. The secrecy was so good that although nearby residents were aware of the blasting they never knew the intention of the project. Even world famous mystery writer and a local resident, Dame Ngaio Marsh did not know what all the blasting was for. She mentions it in her autobiography “Black Beech and Honeydew” (Boston: Little, Brown 1965) "We could hear and feel blasting under our hills where, it was rumoured munitions were being secreted."

During the blasting two railway lines were built at the mouth of the tunnels both for access and to remove the waste soil generated from the blasting. So much soil was removed from the hillside that when dumped the soil filled portions of a nearby valley and even created cliffs of up to 30 ft. Located just over 100 ft below ground the three main caverns of the bunkers together form a U shape. The largest of the caverns is a massive 7 m high, 10 m wide and about 30 m long. The main entrance to the caverns for officers was via a set of stairs that descended 100 ft from the cellar of the Cracroft Wilson homestead. Staff of other ranks were to enter the bunkers through access tunnels in the side of the hill.

The caverns of the bunkers were to be lined with reinforced concrete. Flt Officer Andrew Gray was stationed at Southern Group headquarters for a time and remembers the prefabricated concrete linings arriving slowly up the hill from Mt Somers on Burnett's Transport trucks and being lifted into place by hydraulic equipment. (See Appendix 3 for drawings.) The concrete linings are also thought to be the first examples of pre-stressed concrete made in New Zealand.

The proposed layout for the bunkers was to be similar to Winston Churchill's war cabinet rooms in London. Plans for the layout included a series of rooms including a "plotting room", a "cypher room" and a "combined ops room'' etc. The plans also detailed kitchens, men's and women's toilets, and ventilation and communication systems. All this suggests that Southern Group were preparing to be equipped for a long stay underground.

By 1944 the bunkers were nearing completion but work came to a sudden halt due to changing fortunes in the war. The Japanese invasion seemed unlikely, meaning that Southern Group and the bunkers at Cashmere were no longer necessary. The Public Works Department had been working on the bunkers for a little over a year and a half at a total cost of £35,204.

Some military use was made of the caverns; for example, a local resident, Bernie Bicknell, was a radio operator in the caverns during the war.

Although the Cracroft Caverns were never to be completed, their story does not end there. By December 1944 the Army and the Navy had left Southern Group and the Cracroft Wilson estate. The weekend before the Air Force were due to give the homestead they had rented back to its owners the magnificent building was burnt to the ground. The circumstances leading to the fire have been the cause of much controversy over the years and the true cause of the fire may never be known. A home for the Cracroft Wilsons was rebuilt in ferro-concrete.

Over the years the Cracroft Caverns have also been the site of romance. Mr John Erikson was based for a time at Southern Group headquarters and it was there that he met his bride to be Audrey Erikson who was stationed at Southern Group as a W.A.A.F. Rumours also abound that for a time the empty but not adequately sealed caverns were a favourite spot for romantic rendezvous involving staff from the nearby Princess Margaret Hospital.

Following persistent disturbances by the public, Mr Bill Wilson, of the Cracroft Wilson family who owned the land, sealed the entrances to the Caverns over 45 years ago. The Caverns lay quietly, unknown by the majority, forgotten by a few and kept a secret by others until their rediscovery in 1987. Jeff Field was working as a reporter for TVNZ when he attended a function in 1988 at Princess Margaret Hospital for his mother who was retiring from the nursing staff there. Out of habit he asked a gardener what was going on at the hospital. After a little persuasion the gardener changed his story from, "oh nothing'', to mention the existence of the caverns. Titillated by this piece of information Jeff started a quest for information on the Caverns. An enquiry to the Ministry of Defence led to approval of the use of their library, in which Jeff found a newspaper cutting from the Christchurch "Press" during the mid-war time describing the cavern, and even giving a photograph of the drilling machine. This rather prejudiced an important war secret, and the matter was then hushed up! The Ministry refused to give any further information.

Jeff, by now promoted to Chief Reporter, then passed the assignment on to Bill Cockram, who contacted Mr Bill Wilson of the Cracroft Wilson family, who himself had bulldozed earth to cover the entrances and had a copy of the military map. With the help of the local speleologists, they cracked open the concrete slab sealing a 60 ft ventilation shaft and abseiled down with a filming crew to get the television footage that was screened on "Mainland Touch" in 1988. To their amazement they found the partially completed caverns in good condition, right down to a pair of mouldy workman's boots. L.J.Brown and J.H.Weeber, "Geology of the Christchurch urban area" p. 61 mention the cave; T.Williams and B.Chapman, "Underground tank N0.4, Christchurch" have a complete article in New Zealand Speleological Bulletin Vol.8, N0.148, pp.211-216, Dec.1988.

Jeff Field and Bill Cockram were not the only people stimulated by the discovery of the secret caverns. Morrie Poulton, a technician with the University of Canterbury was desperately searching for a new home for the University's ground-breaking ring laser experiment when he saw Bill Cockram's television footage of the caverns and recognised the answer to the University's problems. The ring laser experiment started off as a collaboration between the University of Canterbury's and Dr Hans Bilger of Oklahoma State University's Professor Hans Bilger this later turned into a collaboration with Munich technical university and the Federal Institute for Applied Geodesy in Germany.

The ree-discovery of the Cracroft Caverns with its stable environment over 100 ft below the ground was the ideal solution to the University's problems and a tailor made home for the ring laser experiment. Another reference to our work is in a chapter on this use of the cavern is in Mark P Silverman's book "And yet it moves: strange systems and subtle questions in physics." (Cambridge UK: Cambridge 1993)

The Cracroft Caverns are now owned by the Christchurch City Council following the subdivision of the Cracroft estate in 1995. In the near future it is planned that for the first time ever the public will be restricted access to the caverns on scheduled open days. It is important that this unique feature of Christchurch's and New Zealand's history is not forgotten again and with it the memory of those who created this unique structure.