Into a cave system France was first explored by humans 8,000 years ago, but how they got in remains a mystery to scientists.
The 40-mile-long underground cave system of Saint-Marcel near Avignon, France, was inhabited by humans, but questions remain about how they carried out the journey.
Even modern, well-equipped explorers have difficulty overcoming obstacles in deeper parts of the caves.
Equipment such as powerful lighting, high-tech safety equipment and advanced equipment are all available now, but this was not possible in the past.
The discovery of broken stalagmites almost a mile beyond the entrance indicates the presence of humans, according to researchers led by geomorphologist Jean-Jacques Delannoy.
Cave explorers broke off pieces of the cave and kept them as trophies.
Jean-Jacques said PNAS: 'This discovery and the fact that the buildings are about 8,000 years old is exceptional. This raises the question of the knowledge of caves in that prehistoric period, their ability to explore and cross shafts, and their control of lighting.”
In addition, scientists were able to find out when the stalagmites were broken by conducting uranium-thorium tests.
The earliest broken point is from 10,000 years ago and the most recent is from 3,000 years ago, while a structure appears to have been built deliberately 8,000 years ago.
The broken structures were first found more than 100 years ago but were ignored by researchers, who assumed they had been left behind by the first tourists to visit the cave in the 1800s.
Researchers write in the paper: 'The evidence for prehistoric human activity in the Saint-Marcel cave is compelling.
'Our research results change the way we look at the Saint-Marcel cave network, giving them a cultural dimension linked to its prehistoric use… the results of the Saint-Marcel cave invite us to take a new look shed light on these societies, their use of caves, which has hitherto been considered limited to the entrance areas, their involvement with deep subterranean landscapes and the associated symbolic dimensions.'
However, the question still remains: why humans broke down and arranged these stalagmites?
Jean-Jacques added: “These are questions to which we will never have answers.
'Like why people started painting in caves and in the deepest areas. We no longer have access to their thoughts.'
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