I n the last decade, considerable attention has been paid to the deterioration of the caves that house the world’s most prominent Paleolithic rock art. This is exemplifi ed by the caves of Lascaux (Dordogne, France) ( 1) and Altamira (Cantabria, Spain), both declared World Heritage Sites. The Altamira Cave has been closed to visitors since 2002. Since 2010, reopening the Altamira Cave has been under consideration. We argue that research indicates the need to preserve the cave by keeping it closed in the near future. In the 1970s, Altamira Cave sparked a political dispute between local and regional administrations and the state and received a great deal of public attention. In October 1977, Altamira had to be closed to the public because of severe deterioration of the paintings after decades of visits (e.g., 175,000 visitors in 1973). In 1978, the Spanish government took over ownership of the cave, which today belongs to the Spanish Ministry of Culture and is managed by the National Museum of Altamira ( 2). In 1982, after a microclimatic survey, Altamira reopened to the public with a limit of 11,000 people per year ( 3) or, in another document, 8500 visitors per year, excluding guides ( 4). However, in September 2002, Altamira had to be closed to the public because of the presence of phototrophic microorganisms on the paintings ( 5) (see the fi gure and fi g. S1), a phenomenon similar to that suffered by Lascaux 50 years before. Colonization by these microorganisms was a consequence of decades of use of artifi cial lighting in the famous Polychrome Hall and was accompanied by development of white microbial colonizations directly on the red paintings ( 6) (see the fi gure and fi g. S3). This showed that Altamira Cave might be mimicking the deterioration processes at Lascaux. Harmful Impacts of Visitors The current status of the cave is the result of the accumulation of multiple microenvironmental changes and impacts suffered from the time of its discovery in 1879 ( 2) (fi gs. S1 to S6). In its natural state, the cave should be an oligotrophic (nutrient-poor) environment with very little connection to the outside atmosphere. The numerous conditioning projects, changes in the top soil and cave sediments, archaeological digs, and massive amounts of visitors transformed the pristine ecosystem into one with an abundance of available nutrients. Preservation of a large part of the Polychrome Hall ceiling paintings from their creation more than 14,000 years ago until their discovery was aided by several factors: absence of light; low rates of water infi ltration, precipitation of speleothems (mineral deposits) and exchange with the external atmosphere; and the maintenance of very stable microenvironmental conditions because of limited airfl ow in the Polychrome Hall. The Spanish National Resource Council (Consejo Superior de Investigaciones Científi cas, CSIC) studied the cave between 1996 and 1999. The studies focused on determining the impact of visitors on the micro-environmental conditions of the Polychrome Hall and highlighted the need to revise the criteria used by the University of Cantabria to design the 1982 visitation schedule ( 7) because deterioration processes had not stopped. Sanchez-Moral et al. ( 3) concluded that corrosion caused by visitors under the 1982 visitation schedule would be up to 78 times that arising from natural processes. Phototrophic microorganisms were observed in 2000 in the Polychrome Hall, as resting forms or with their metabolic activity reduced to a minimum. Cañaveras et al. ( 8) predicted that in the event of a change in the lighting conditions—such as permanent or extensive lighting, as subsequently occurred—growth of these communities would take place. This did indeed occur in 2002 (fig. S1A) and led to the second cave closure. Once the cave was closed in 2002, the Ministry of Culture appointed the CSIC team to survey and control the cave from 2003 to 2005 and from 2007 to 2009. Thereafter, CSIC is controlling cave microclimate through successive contracts, the last ending on March 2012. Research focused on the cave as an ecosystem dependent both on microenvironmental and geochemical characteristics of the underground environment and an energy-matter exchange with the external atmosphere. The conclusions of the research were against the reopening of the cave to visitors ( 9– 11). This was supported by environmental, geochemical, hydrological, and microbiological data put at the disposal of the Ministry of Culture.