Like the cave paintings at Lascaux, the ancient necropolis beneath St. Peter’s Basilica has suffered decades of damage caused by, of all things, human breath.

The moisture in human breath combines with salts and minerals in rock, creating a corrosive mixture that eats away at painted walls and frescoes. At Lascaux, authorities were forced to close the cave to visitors in 1963 to avert further damage. Vatican officials, on the other hand, have launched a conservation program that should keep the necropolis open and accessible for years to come.

After several years of monitoring the site’s atmospheric conditions, engineers determined that the moisture problem was aggravated by the artificial lighting that had been installed to promote tourism. Not only did the lights aid the growth of algae, which are sustained by photosynthesis, but heat from the lights encouraged the spread of fungi and bacteria, which were attacking the frescoes and mosaics and staining the mausoleums’ walls.

After trying to solve these problems piecemeal, the engineers finally developed a sweeping conservation plan that has reduced some of the most harmful effects caused by opening up and illuminating the dark City of the Dead.

They lowered the temperature in the necropolis, reducing the amount of moisture in the air. They also dimmed the lighting and installed filters to minimize the radiation that had nurtured the growth of algae. Now, automated switches turn off lights as soon as visitors exit certain areas, particularly near the vulnerable frescoes.

This new system was designed to be as unobtrusive as possible. The lights and ventilation are nearly silent, and spotlights are carefully positioned to illuminate only the mausoleums and their contents (and not modern steel beams and reinforced concrete). The necropolis thus retains a kind of hushed, reverential beauty, befitting the spot where St. Peter’s bones are reputedly entombed.