The vast cavern complex in the Cantabria region of northern Spain is covered in paintings of animals dated to between 14,000 and 20,000 years ago.
     For the past 12 years, visitors have had to settle for a replica in a museum, but now small groups of visitors are again being allowed into the cave as part of an experiment to determine whether the paintings can support the presence of sightseers.
     Until August, on a random day of the week visitors will be invited to enter a draw, and five chosen for a guided tour including 37 minutes inside the cave. They will put on special suits, masks and shoes before entering.
     Researchers will measure their impact on the cave's temperature, humidity, microbiological contamination and CO2 levels. The results will be used to determine whether or not the cave can be reopened to the public, a controversial decision that has pitted the local tourist economy against government scientists.
     The site has been closed several times, starting in 1977 after scientists warned that body heat and CO2 levels from the 3,000 daily visitors were deteriorating the paintings. The cave was again closed to the public in 2002 after scientists blamed body heat, light and moisture for the appearance of green mould on some of the paintings. Since then, the regional government has been lobbying for the site to be reopened, against the recommendations of the government's main research body. A 2010 report made it very clear that the cave shouldn't be open to visitors, with lead researcher Sergio Sánchez Moral recently warning: "The consequences of doing so are immeasurable."
     José Antonio Lasheras, director of the Museo de Altamira, defends the decision. The tours, he says, are part of a carefully calculated equation to find a balance between conservation efforts and making the country's heritage as accessible as possible. "It's a controlled risk," he says.

Edited from The Guardian (26 February 2014)