After more than a decade closed to the outside world, Spain’s Altamira cave complex, which contains some of the most outstanding examples of Paleolithic art anywhere on the planet, is to reopen. Between now and August visitors will be chosen at random to be allowed into the caves, which are situated in the northern region of Cantabria, as part of an “experiment,” to see whether the site can be reopened to limited numbers, say Altamira’s board of directors.

The main chamber at Altamira, which was declared a Unesco World Heritage Site in 1985, features 21 bison painted in red and black, which appear to be to charging against a low, limestone ceiling. The paintings are estimated to be between 14,000 and 20,000 years old.

In 2002 the cave complex was completely shut off to most visitors after scientists detected green mold stains on the paintings in the main chamber. An exact replica of the caves and its paintings was created within a museum a few hundred feet from the original near the town of Santillana del Mar.

The Altamira board of directors says its goal is to provide limited public access to the cave, but without further damaging the paintings. “What we have done is to agree on an experiment, which is part of the conservation program,” says José Antonio Lasheras, the director of the Altamira National Museum and Research Center. “We have been evaluating the impact of researchers on the caves, and we thought that we could do this just as well using outside visitors,” says Lasheras.

In 2010, researchers said that even the motion of visitors moving through the caves could prove disruptive, stirring the air and encouraging the release of mostly dormant bacterial and fungal spores. This could lead to the growth of new microorganisms, which for now seem to be slowing their advance.

From February of this year, five museum visitors a week will be chosen at random and offered a guided tour. “But anybody coming into the caves will have to wear the appropriate clothing and follow certain procedures,” says Lasheras. “They will be required to fill in a questionnaire afterwards, and will then take part in an anthropological survey.”

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