The French government is taking emergency action to rescue the celebrated cave paintings of the Lascaux caverns from a fungus.
Archeological experts have begun applying a fungicide to halt the spread of grey and black mould in the caverns, dubbed the Sistine Chapel of prehistory.
The caves, discovered in 1940 by teenagers walking a dog, contain images of bulls, deer and horses believed to be 15,000-17,000 years old.
The French government has closed the caves located about 450 kilometres south of Paris to everyone, including scientists and historians, for three months and will replace an air circulation system that may be partly responsible for the fungus.
The system, installed seven years ago, may have been poorly designed, as a similar fungal attack took place after its installation.
The fungus, which grows because of moist conditions, is threatening some of the 600 drawings in yellow, red and black mineral pigments that cover the caves.
The drawings, believed to have been painted by hunter-gatherers, have survived since the last Ice Age.
A team of specialists who assessed the site before Christmas recommended stopping all activity in the caverns and taking action to stop the fungus.
They put pressure on the French government by alerting UNESCO, which classifies the caverns as a World Heritage Site, about the conditions.
Laurence Leaute-Beasley, president of the International Committee for the Preservation of Lascaux, called for the management of the caves to be taken out of the hands of the French government, saying someone who understands the science involved should take over.
The French government, not wanting such an an important site to be seen as neglected, has decided to accept the committee's advice and act now against the fungus.
The experts disagreed on the cause of the problem. Some say global warming is to blame, others that human activity in the caves is exacerbating the problems.
One of the projects to be halted by the emergency treatment is a survey that was to make a three-dimensional digital record of every painting in the caverns.
The caves, which had been a major tourist draw in the 1950s, have been closed to the public since 1963.
In 2001 and 2002, a white fungus spread over much of the caves, but was successfully brought under control.