A competition to document prehistoric art in a coastal cave in France

When prehistoric artists entered the the narrow first passage of what is now Kosker Cave about 27,000 years ago they moved from the flat grassy plains to the slight slope to the foot of the cliff a few miles from the sea. Today, when Bertrand Chazaly enters a French cave to digitally scan the paintings and prints created by these artists, he faces a more difficult journey.

High sea levels from the last ice age left the mouth of the cave more than 120 feet underwater and submerged a tunnel that is longer than a football field. But there are two chambers at its end that are at least partially dry — for now.

Kosker Cave contains a treasure trove of prehistoric art, but its inaccessibility makes it difficult to explore. And the clock is ticking: the rising sea level is washing away paintings and engravings. The 3D digital models created by Chazali’s team will preserve the site virtually, but data collection requires technical innovation and physical endurance.

“It’s really hard,” says Chazali, a surveying engineer who specializes in speleology and cultural heritage at geodata company Fugro. For the past 12 years, he has worked in the cave for several weeks each year. “It’s really a matter of passion.”

This year, Chazali’s team completed mapping the parts of the cave that are above water, using laser scanning and photogrammetry to create models with an accuracy of 0.1 mm. Next, they will document what is under the water. Although no art survives on the submerged walls, a complete model of the cave system will help researchers understand how it was used in prehistoric times.

The art left in the cave, which dates from 19,000 to 27,000 years ago, includes hand stencils of men, women, and children, as well as numerous animal figurines of bison, horses, goats, and chamois, as well as seals and seabirds.

Today, the entrance to Cosker Cave is more than 120 feet underwater at the base of this cliff. Georges Seguin (Ocky), CC BY-SA 3.0/Wikimedia

Prehistorian Jean Clotte was working for the French Ministry of Culture in 1991 when diver Henri Cosker reported a cave he had discovered a few years earlier on the rocky coast east of Marseille. Kosker’s photos intrigued Clottes, but he couldn’t see it with his own eyes—he wasn’t a diver. Even now, exploring Cosquer in person requires a special skill set that combines experience in both cave art and cave diving. “Not many people can do that,” says Clotts.

Clotts eventually became one of those people, learning to dive at the age of 69 just so he could explore Cosker Cave. He has made dozens of visits and, having reached the dry chambers of the cave, says that his work is similar to that of other prehistoric sites. But just getting to the cave can be difficult; rough seas and bad weather often cancel outings.

Cosquer presented unique challenges even for Chazaly, who worked in some extreme locations, including the spire of Mont Saint-Michel, 450 meters underground in the nuclear waste storage galleries, and the Great Hypostyle Hall of Egypt’s Karnak Temple. When the French Ministry of Culture first contacted him more than a decade ago about the possibility of creating a digital model of the Cosquer Cave, Chazaly first needed to improve his diving skills, from recreational to professional. It then took several years of test visits before his team was able to begin scanning the cave in 2017.

Each scan involves one or two researchers accompanied by two expert divers who are familiar with the cave. From a boat off the rocky coast of the Calanque National Park, they descend into the turquoise waters of the Mediterranean Sea, carrying equipment in sealed bags and a key to unlock the tunnel entrance gate.

A cross-section of Kosker Cave today as the water level continues to rise in both chambers.
A cross-section of Kosker Cave today as the water level continues to rise in both chambers. Courtesy of Bertrand Chazaly

With one diver in front and one behind, they swim slowly, careful not to kick the sediment on the floor, which, if disturbed, dangerously clouds the water. In 1991, three divers died after they became disoriented by sediment that rolled up the passage. Now the length of the tunnel is extended by a lifebuoy.

After about 15 minutes, the group climbs on beach, a beach where they leave their diving gear to work above water level on a slippery floor among stalagmites and stalactites. Regardless of the season, Chazali says, the temperature is always around 64 degrees Fahrenheit.

There are strict rules inside. They should not touch fragile walls. The only food source allowed is water. They cannot have a snack during a long stay. They can’t pee. “You really have to be prepared,” Chazali says. “Sometimes you’re in a hurry to get out of the cave.”

Chazali’s team set up a laser scanner for a remote cave in low-light conditions. Although the scanner records 70 million points in three minutes, positioning the device and people took a long time. During each scan, the team hid so they wouldn’t show up in the data. At times, colleagues physically kept Chazali in an unreliable dry place. Sometimes he stood up to his chest in water with the device on a tripod. Twice the scanners fell into the water and were destroyed.

Chazali (left) and another team member discuss how best to position scanners and other equipment in a partially submerged chamber.
Chazali (left) and another team member discuss how best to position scanners and other equipment in a partially submerged chamber. © Luc Vanrell–Immadras/courtesy of Bertrand Chazaly

Examining each dry crevice of the cave, the lights illuminated new details. Shazali remembers how once an archaeologist with his team shouted: “Look, there’s a little horse!”

“Almost 30 years after the discovery, just to find a new engraving or something like that is pretty amazing,” he says, “but it happened.”

New challenges will arise this year as Chazali’s team begins documenting submerged parts of the cave, including those that have never been explored before. Divers will spend longer periods of time underwater, which involves greater physical risk. The team will need to move the diving equipment through the narrow passage between the chambers, rather than leaving it at the water’s edge without touching the walls, which are easily damaged. To document the long tunnel entrance, where it is impossible to stop due to the risk of disturbing the sediment, Chazali’s team designed a ring system of cameras and lights that will record 360-degree images along the way.

Scientists familiar with the site see great value in a digital replica of the cave, “a bubble of memory lost beneath the sea,” according to geologist and prehistorian Jacques Collin-Girard, who says his visits to the cave in the 1990s were similar to Herbert Wells or the adventures of Arthur Conan Doyle. Surface scanning has already been used to create a replica of the cave for the new Cosquer Méditerranée museum in Marseille. The digital model, Collina-Girard says, makes the cave accessible to the public and researchers from afar and preserves what the sea will soon take away.

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