In a cave accessible only via a deadly submerged passage, archaeologists and 3D scanning experts have worked together to record some of the earliest artworks in Europe. In nearby Marseilles, these scans have been used to replicate parts of the cave for the public to view.

On a late summer day in 1991, Henri Cosquer was preparing for another dive, 40m below the surface of the Mediterranean. He had first spotted the opening to a cave in 1985, and then, little-by-little, explored the narrow 120-metre passage leading up from it.

In 1992, Cosquer described his exploration of the cave, in a paper written with Jean Clottes and Jean Courtin. Courtin is a leading expert in cave art, and Jean Clottes was the initial leader of research on the cave. Clottes later recruited Luc Vanrell, a celebrated underwater archaeologist to help explore the cave.

Access to the cave is extremely dangerous, they wrote. As divers swim their way through the passage, their fins disturb the sea bed. It darkens the water to the point that, even with powerful lamps, they cannot see their own hands.  In the summer of 1991, three divers from Grenoble had got lost in these darkened passages, panicked, and drowned.

But later in the same year, Cosquer reached the main chamber of the cave. On subsequent dives, he and his fellow divers would see something no-one had seen since the rising waters of the Mediterranean drowned the cave entrance: signs of prehistoric wall art.

Luc Vanrell joined the team exploring the cave a little later, and has spent many years leading work on the project. He had already made his name in French underwater archaeology, by conducting dives that helped identify the plane in which Little Prince author and pilot Antoine de Saint-Exupéry had lost his life.

Even for an experienced diver like Vanrell, this was a strenuous dive which required additional training. But the experience of entering this hidden, ancient, space made it well worth the risk. “The first visit to the cave feels mysterious, because the underwater journey, and especially the underground journey to access the main chamber, is very long. The sense of mystery only progresses along the way,” he says.

“And when you arrive, the cave’s mineral structure is absolutely magnificent. So the first shock is really an aesthetic response to the mineral beauty of the cave. And then you are confronted with the almost physical presence of some of our earliest ancestors.

As Cosquer and his fellow divers had first explored the caves though, they were not immediately aware of the art. When they first saw markings on the walls, they were not sure what they were seeing. They returned with a camera, took photos, and began to share them with scientists like Clottes and Courtin.

“Some of the archaeologists were really not sure it was prehistoric drawings. They thought maybe it was not very serious,” says Bertrand Chazaly. “So they really had to wait for the expertise of one of the big parietal [cave art] specialists in France, Jean Courtin, who carefully looked at the picture and said, ‘No, no, that’s true art, actually’. And they were able to bring Courtin into the cave so that he was able to look at the drawings and just say, ‘Okay, that’s really, really, parietal art’.”

Chazaly is a digital and 3D scanning expert with Fugro. He has developed techniques to combine dense 3d scans with photogrammetry. A pioneer of these tools in France, he has worked for UNESCO in Egypt. The tools he uses can also be used in industry, to map mines, for example. He had experience as a leisure diver, but had never dived in such dangerous conditions as these at Cosquer. When the French ministry of culture came looking for an expert to map the caves, he first had to develop his diving skills.

And then he had to develop a plan to map the caves.

“They really wanted someone, or company, to get the most accurate 3d model of the caves, saying that they really needed three levels of details. So, first level was to get the general volumetry of the cave and the surfaces located above and below water.  Then the second level of detail was to get a detailed 3d point cloud, where the engraved and painted surfaces could be located to geo reference these graphical entities.  And then they asked for a last level of detail, which was the finest 3d modelling of the graphical entities as possible at the level of details, that could allow a complete and deep study of the engraving on the computer. So these three steps were quite clear, but what was not clear is the way we could deliver these three steps. 

“The answer was to mix different technologies. The first one is 3d scanning. So you use the 3d scanner, which is a tool that you put on a tripod, and it’s really scanning all the surfaces in a 360 degree field of view, with a density and an accuracy of about two millimetres, which means that the instrument is measuring 3d points, one point every two millimetres, everywhere.

“The problem is that if you have a wall or something in front of the scanner, everything that is behind this wall, you can’t see it. You have to record several scan positions, so that by the end when you mix these different scan positions, you have been able to cover all the walls that you have in the cave. And that was one of the challenges, which was that I needed to record enough scan positions to provide a 3d model that could cover all the surfaces of the cave.”

Chazaly’s 3d scans could deliver the first two of the scientists’ requirements. They showed the shape of the cave, and the textures of the surface. But they were still not detailed enough to allow Clottes and Vandrell’s team to study the art properly.

“To reach this last level of detail, we changed to another technology, which is photogrammetry,” says Chazaly. “Photogrammetry involves taking pictures of an object from different points of view, with overlapping coverage between pictures. And then software is able to recognise in two different pictures the same points that have been photographed, and to reconstruct the 3d shape of the object that has been photographed. 

“If you use a very high resolution camera, and if you take pictures very close to the object, then you are able to provide a 3d model that is very, very dense and very accurate. In Cosquer, we have been taking pictures at less than one metre distance from the surface. So that the final resolution of the picture was better than 0.1 millimetre. All the engraved and painted surfaces of the cave have been photographed with more than one pixel every 0.1 millimetre at the surface of the cave. It’s a huge set of pictures, several terabytes. But then by computing these pictures, we were able to reconstruct the surface at a resolution where we can see all the very thin engravings.”

The detailed 3D scan that Chazaly constructed opened new ways of working for Vanrell and the wider scientific community. “Getting down into the millimetric grid with Bertrand has been an exciting project. It was really something great, both in terms of the technical and scientific adventure, and the human adventure as well,” says the underwater archaeologist.

“We have been able to use precise and exact measurements in the laboratory, which would have been very difficult in the cave. The job now is to contextualise what we see in the laboratory, which was possible thanks to the quality of the 3D results. And there is the possibility of collaboration, of sharing detail with other scientists thanks to the 3D model, which would not be possible with a simple film.”

The scans of the cave have allowed Vanrell and his team to study the cave in previously impossible precision. It has allowed them to share its hidden secrets with scientists around the world. But it has also allowed them to bring an experience of the cave to the wider public.

To do this, the local council, Region Sud, recruited Alain Dallis, an artist who has recreated cave art from across France. Growing up in Lascaux, one of the first sites where prehistoric wall art was discovered, It’s work he was born to do.

“For each cave, I used different artistic techniques, as the methods of measurement evolved,” Dallis explains. “I had to adapt to the evolution of new scientific tools. With the use of 3D scans, I was able to develop new digital ways of working, using a five-axis milling machine to recreate the cave walls.”

The reconstructions and interpretations Dallis created are now on show at the Villa Méditerranée in Marseilles. Vanrell emphasises the importance of bringing the re-creation to the public. “I have thought for a long time that reproducing the art of the cave is an important part of sharing scientific knowledge with the public. The Cosquer cave, with its cold waters, is such a difficult place for us to get to. It has taken the discipline of some serious explorers, like Bertrand, and a spirit of human adventure, to recreate the caves in 3D.”