More recently, they found the skeleton of a man who died 6000 years ago, then the body of a shepherd murdered for a guinea and then thrown down a daylight hole into the darkness. The bushranger Ben Hall once hid in the same pitch-black.

But the Cliefden Caves will go under water again if a proposal for another dam across the Belubula River near Canowindra gets the nod, collateral damage in a battle between the preserved past and the preferred future.

Proponents claim the dam would also help save the Murray Darling system but the cave system's extent is so unknown that opponents wonder if the the water may just  go down a gigantic plughole in the Earth.

The first anyone knew about flooding the caves was last January.

That's when the National federal MP for Calare, John Cobb, called for construction of a dam and announced he had written to everyone – the  Prime Minister, Deputy Prime Minister, Water Minister – pushing the project.

Mr Cobb said his proposed $150 million Needles Gap dam near Canowindra would have a storage capacity of up 100,000 megalitres, provide greater water security for urban, irrigation and mining needs in eight local government areas – Bathurst, Blayney, Orange, Cabonne, Cowra, Parkes, Forbes and Grenfell – and take five years to build while providing 150 jobs.

He said the central west had lost many jobs and needed a confidence boost: "We need this Needles Dam project now more than ever."

Then the Baird government allocated $4.5 million in the June budget for a dam feasibility study due for completion next month.

But on Tuesday, more than 100 people attended a public meeting in Orange and gave the proposal the thumbs down.

The NSW upper house Greens MP Jeremy Buckingham said the dam did not rate a mention in a 2009 water security municipal study.

"This is a back-of-the-beer-coaster proposal for a dam from a National Party that is stuck in the 1950s," he said.

Europeans first recorded the Cliefden Caves in 1815. The limestone, from the Ordovician age, was the first discovered in NSW and the 100 or more caves teem with fossils. They are isolated, on private property but not protected by statute ,although the National Trust recently nominated their addition to the NSW State Heritage Register.

Bruce Welch, of the Save Cliefden Caves Association, said several fossils occurred nowhere else in the world.

"In terms of modern biological conservation, the known population of Belubula spectacula – about 20 individuals at most – can be argued to be equivalent in scientific importance to the discovery of the Wollemi Pine," he said.

"The dam proposal is as senseless and environmentally destructive as the flooding of the Pedder in Tasmania. Flooding these caves is like resuming whaling – there are always more jobs, tourism and economic benefit in preserving, exploring and treasuring nature than destroying it."