Conservationists at Persepolis, Iran’s most iconic ancient site, are waging a delicate battle against an unlikely adversary: tiny but persistent lichens eroding the millennia-old monuments.

The fight, which began years ago, is aimed at stopping the threat to the integrity of the site’s structures and its intricate carvings from lichens, organisms that grow on surfaces like stone and can slowly break them down over time.

Built in the 6th century BC by Darius I, Persepolis has withstood destruction, looting, earthquakes, fires and harsh weather. It remains a source of pride for Iranians and a major tourist destination.