The 4.5 km Ruakuri Cave at Waitomo was re-opened 10 years ago this month.
It has unique disabled access, thanks to a project which installed a 15-metre-deep spiral entrance and a suspended walkway through much of its length.
Members of the construction crew responsible will gather at Waitomo in October to mark its 10th anniversary.
About 50 cavers tackled the tricky renovation, which followed the cave being closed for 18 years due to a financial dispute.
Ruakuri operations supervisor Angus Stubbs says most of them still work in adventure tourism, either in New Zealand or overseas, though probably only about half can attend the reunion.
During the early stages of the work, these men and women had to swim to work in the dark each day.
Wearing wetsuits and clutching waterproof toolbags, they'd jump down a subterranean waterfall, then ride currents to construction sites in the bowels of the earth.
"They even 'jumped' a concrete mixer over the falls, floating it to about the middle of the 80-metre-deep system.
"Our crew wheelbarrowed tonnes of concrete through the cave. They built the suspended walkway amidst fragile protected structures and coped with cold and dark. Being experienced they handled everything well, and injuries were few."
Today there's a gentle caving experience for less mobile visitors, through to scary black water rafting, or even hurtling down flying foxes in the dark.
Two streams flow through the cave system, which has several entrances.
Mr Stubbs says part of the fun is coming upon other tourists or adventurers, "whom you did not enter the cave with."
Cave's rich history
According to Maori legend, Ruakuri Cave ('rua' meaning den, and 'kuri' meaning dog) was discovered 400-500 years ago by a young Maori hunting birds. He was attacked by wild dogs just outside the original cave entrance, but survived to reveal the location to his tribe.
The cave was first opened to visitors in 1904 by James Holden, an ancestor of the family that still owns much of the land above the system.
It was the second cave in the region to be opened as an attraction and was quickly pronounced, "visually impressive . . . an experience almost spiritual".
The Government claimed ownership of Ruakuri Cave and the attraction was then operated by the former Tourist Hotel Corporation until February 1988, when a legal and financial dispute forced its closure.
Ruakuri Cave was closed for over 18 years but officially re-opened for underground guided walking tours in July 2005. This followed extensive redevelopment by Tourism Holdings Limited and an agreement with the Holden Family Trust of Waitomo. There were 18 months of underground construction work on walkways, bridges and a new spiral entrance way.
Taking the tour
* The journey begins with a descent via the spiral entrance and continues among folding shawl-like limestone formations, undergound waterfalls and glow worms. New Zealand's longest guided underground walking tour boasts full wheelchair and pushchair access.
* This, the 25th longest cave in the country, is fed by substantial Huhunui and Okahua streams.
* The entrance spiral provided an alternative to the old entrance, which was an urupa (Maori burial sight). Perhaps resembling a nuclear bomb bunker entrance from the 1960s, the design demanded removal of 300 tonnes of earth. Once completed, a horizontal hydraulic tunnel borer pushed into the existing cave passage. The new access created allowed dry and level entry for workers, who no longer had to swim to work.
* Today four tours use walkways, streams, dry passages, muddy passages, vertical or horizontal passages.