Imagine slipping into a small nondescript crevasse on an unmemorable hill, falling 10 metres in the darkness until you crash into the floor.

You would be either dead or gravely injured, writhing in agony with multiple broken bones as a light shines in the darkness far above you.

That gives you an idea of the last moments of thousands of animals that have perished near Martinborough over the past few thousand years and, most recently, a few unlucky sheep.

ALAN TENNYSON A Te Papa staff member descending into the cave.

The cave in the ground near the Wairarapa town contains one of the richest deposits of extinct bird bones ever discovered in New Zealand.

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Te Papa curator of vertebrates Alan Tennyson said the cave was first discovered in 1914 by a deerstalker who alerted the former incarnation of Te Papa, the Dominion Museum.

RACHEL HOCKRIDGE Some of the bones recovered from a cave described as one of New Zealand's richest fossil sites.

Tennyson went back to the site earlier this year and gathered a few more samples but added that the original expedition by Dominion Museum in 1920 had gathered thousands of samples that Te Papa staff were still categorising.

"This cave is one of the most important fossil sites in New Zealand, really useful and actually well known among bird palaeontologists. Most of the bones are a few thousand years old. The further into the mud you go the older they get.

"There are still a lot of bones packed into the mud at the bottom of the cave. The area historically would have been covered in bush, teeming with animal life so even if only three animals a year fell, in 5000 years you would have 15,000 sets of bones down there.

RACHEL HOCKRIDGE A tuatara jawbone found in the cave.

"There are the remains of moa, kiwi, north island takahē, Finsch's​ duck, weka, kākāpō and it is also the best known site for the extinct adzebill, (a large flightless bird that stood about 80 cm high weighing up to 18 kg) it also has tuatara bones, extinct native frogs and many other animals."

Tennyson said the site would be left undisturbed as a resource because it was safe from the elements and would remain relatively unchanged over time enabling it to be examined again in the future.

Bones from the site could still provide a lot insight into the lives and behaviour of the birds and other animals and in future as technology progressed more could potentially be revealed, he said.