8,000 years ago, people navigated deep into a dark French cave. The question is: how? : ScienceAlert

Our world is littered with fascinating, dangerous, but beautiful holes in the silent depths of the Earth’s crust. To explore a cave is to encounter awe – a calm, almost alien world, far removed from the chatter and bustle of the world above ground.

One of the most spectacular known cave networks in the world is the Saint-Marcel Cave in France. The entrance area has been inhabited by humans for thousands of years and dates back to the Middle Paleolithic.

But there is much more to it than just the entrance. The cave extends for at least 64 kilometers (40 miles), a winding, convoluted cavity bored through the Earth’s crust.

Due to its long history of habitation, it is of great interest to anthropologists. But now scientists led by geomorphologist Jean-Jacques Delannoy of the French National Center for Scientific Research (CNRS) have found a puzzle.

Deep in the cave, along a dangerous path with deep holes, they found broken stalagmites more than a mile beyond the entrance, indicating the presence of humans – about 8,000 years ago.

Even by today’s standards, which include safety equipment, advanced equipment and powerful lighting, the potholes in the path are considered dangerous. This begs the question: how did ancient people get to the place, and how did they get out again?

“This discovery and the fact that the structures are about 8,000 years old are exceptional,” says Delannoy PNAS. “This raises the question about the knowledge of caves in that prehistoric period, their ability to explore and cross shafts, and their control of lighting.”

The part of the cave that Delannoy and his colleagues studied has been known for some time, with a number of broken rock formations, called speleothems, littering the floor. These are mineral deposits formed by groundwater in caves, such as stalagmites (an upward-facing formation on the cave floor) and stalactites (a downward-hanging formation on the cave ceiling).

It is not uncommon for well-known caves to have broken speleothems. In the late 19th century, it was common for cave explorers and tourists to break off pieces of rock as souvenirs, or leave marks on the cave walls to commemorate their visit. The broken speleothems in Saint-Marcel were believed to be the work of such tourists.

But Delannoy and his colleagues have found ancient traces of human presence in other caves, raising questions about when exactly the rocks in Saint-Marcel were disturbed.

Fortunately, there are ways we can find out with rocks, and this is what the researchers set out to do. Speleothems are created from a long, continuous interaction with water; You can break off a stalagmite at the root, but if the water continues to flow and deposit minerals, that stalagmite will regrow.

The researchers examined regrowth on the fractured formations, but that’s not all. They also analyzed the ratios of uranium and thorium in the speleothems, a technique known as uranium-thorium dating. It works because uranium is soluble in water, but one of its decay products, thorium, is not. So any thorium in a sample is the decay product of uranium after the mineral precipitates.

Because the rate at which uranium decays into thorium is fixed and known, scientists can look at the amount of each in the sample to determine how long it has been since the mineral formed. Using these techniques, the researchers discovered that the speleothems were usually formed between 125,000 and 70,000 years ago.

The team found that the earliest broken point was about 10,000 years ago. The most recent was about 3,000 years ago. But there was another clue. A large number of the broken pieces appear to have been deliberately placed, creating a structure in the room. This structure, the researchers discovered, was created about 8,000 years ago.

There is no doubt about that, the researchers conclude. There were people here long before we thought they could be, somehow navigating the dark, dangerous passage and breaking rocks to build something. How they did that is for future work; soot deposits on the room walls can be a clue if they are installed at the same time as the structure.

“The evidence for prehistoric human activity in the Saint-Marcel cave is compelling,” they write in their paper.

“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.”

Their findings have been published in the Journal of archaeological method and theory.