the swell builds the pressure to protect paintings and engravings from the submerged cavity

From their first forays into Cosquer cave, divers observed on their depth gauges an unexpected phenomenon, confirmed since: the water level in the cavity is always at least 20 centimeters lower than that of the Mediterranean. The difference, variable depending on the weather and the season, can reach 1.40 meters and plays a major role in the protection of graphic works by preserving them from greater submersion.

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The scientific team recently clarified the mechanism at the origin of this phenomenon which calls for the very particular topography of the site and is explained by an overpressure of the air in the cave. The two decorated and partially flooded rooms communicate in an aerial way, via the large well located at the bottom of the cave, with a network of upper galleries which was never visited during prehistoric times. Scientists access it by metal ladders that equip the vertical well.

Air exchanges

Known in the 1990s and fitted out for technical needs, this superior network in turn communicates with the sea by an impassable siphon, where air exchanges take place: “In a south-easterly wind and strong swell, each wave hitting the cliff injects a large quantity of air through this duct. Compressed in the cave, the latter then causes an equivalent drop in the body of water ”, details researcher Bruno Arfib (European Center for Research and Education in Environmental Geosciences), who carried out the study with archaeologists Luc Vanrell and Michel Olive. After such episodes of pressurization, very rapid and placing many works dry, the compressed air slowly escapes from the cavity through cracks in the karst rock, and the water level rises. Rapid drops in the water level are observed during the fall and winter, as well as a slow rise in spring and summer.

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Twice a day, this particular hydraulic operation also plays a protective role against the tide – small in the Mediterranean but real – whose signal in the cave is greatly damped. Thus, measurements showed that a tidal range of 20 centimeters from the incoming tide generated only 8 centimeters of water level rise in the cave. For the preservation of works, it is therefore vital to understand and anticipate the evolution of this exceptional phenomenon.