Crawling through tight underground passages in southern France, paleontologist Jean-David Moreau and his colleagues have to descend 500 meters below the surface to reach the only known footprints of long-necked dinosaurs called sauropods ever found in a natural cave.

The team discovered the prints, left by behemoths related to Brachiosaurus, in Castelbouc Cave in December 2015 (SN: 2/21/18). But getting to the site might make even the most hardened field scientists balk. Wriggling through such dark, damp and cramped spaces every time they visit is challenging for elbows and knees, and even trickier when carrying delicate equipment such as cameras, lights and laser scanners.

It’s both physically exhausting and “not comfortable for someone claustrophobic,” with the researchers spending up to 12 hours underground each time, says Moreau, of the Université Bourgogne Franche-Comté in Dijon. It can be dangerous too, as some parts of the cave are periodically flooded, so accessing the deep chambers must be limited to periods of drought, he says.

Moreau has studied fossilized dinosaur footprints and plants for more than a decade in southern France’s Causses Basin, one of the richest areas for aboveground dinosaur tracks in Europe. When spelunkers chanced upon some underground prints in 2013, Moreau and his colleagues realized there could be lots of dinosaur prints within the region’s many deep, limestone caves. Footprints left in soft mud or sand hundred million years ago could have been turned to rock and forced underground over many eons.

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And deep caves, being less exposed to wind and rain, “can occasionally offer larger and better-preserved surfaces [imprinted by dinosaur steps] than outdoor outcrops,” Moreau says.

Moreau’s team is one of only two teams to have discovered dinosaur footprints in natural caverns, though prints also have been found around the world in human-made railway tunnels and mines. “The discovery of dinosaur tracks inside a natural karstic cave is extremely rare,” he says.

The first subsurface dinosaur prints that Moreau’s team found were 20 kilometers away from Castelbouc at a site called Malaval Cave, reached via an hour-long clamber through an underground river with several 10-meter drops. “One of the main difficulties in the Malaval Cave is to walk taking care to not touch or break any of the delicate and unique [mineral formations],” Moreau says.

Those three-toed prints, each up to 30 centimeters long and detailed in 2018 in the International Journal of Speleology, were left by carnivorous dinosaurs walking upright on their hind legs through marshland about 200 million years ago.

Paleontologist Jean-David Moreau examines a three-toed footprint left behind by a carnivorous dinosaur millions of years ago and now found in Malaval Cave in southern France. Vincent Trincal

In contrast, the five-toed herbivore tracks in Castelbouc Cave are each up to 1.25 meters long and were left by three enormous herbivorous sauropods that walked the shoreline of a sea about 168 million years ago. What’s more, these prints are on the cave’s ceiling 10 meters above the floor, the team reports in a study published online March 25 in Journal of Vertebrate Paleontology.

In fact, “the tracks we see on the roof are not ‘footprints,’ they are ‘counterprints,’” Moreau explains. “The dinosaurs walked on a surface of clay, which is nowadays totally eroded to form the cave. Here, we only see the overlying layer [of sediment that filled in the footprints],” leaving reverse prints bulging out of the ceiling. It’s similar to what you’d see if you filled a footprint in mud with plaster and then washed all of the mud away to leave the cast.

The tracks are important as they hail from a time in the early to mid-Jurassic Period from 200 million to 168 million years ago when sauropods were diversifying and spreading across the world, but relatively few fossil bones have been found (SN: 12/1/15). These prints confirm that sauropods then inhabited coastal or wetland environments in what is now southern France.

Moreau is now leading researchers in exploring “another deep and long cave, which has yielded hundreds of dinosaur footprints,” he says. The team has yet to publish those results, which he says may prove to be the most exciting of all.