Conservationists at Persepolis, Iran’s most iconic ancient site, and the ceremonial capital of the Achaemenid Empire, are having to contend with stone-eating lichens, which have been eroding millennia-old monuments. A UNESCO World Heritage Site since 1979, Persepolis features over 1700-year-old colossal sculptures and intricate stone reliefs of ancient Persian kings, nobles and deities. The aim is to ward off the threat to the integrity of the site’s structures and its intricate carvings by lichens – tiny and microscopic organisms that grow on surfaces like stone, slowly breaking them down, over time.

Built in the 6th century BC by Darius I, Persepolis has withstood destruction, looting, earthquakes, fires and harsh weather. Alireza Asgari Chaverdi, director of the site, about 50 kilometres from Shiraz, refers to it as an “Open-air museum reflecting 25 centuries of Middle Eastern life,” and “the foundation of Iran’s history, culture and socio-cultural life.” Adds site conservationist, Shahram Rahbar, “This is the most serious problem, especially for carvings on stones… these organisms could reduce these relics to dust within 50 to 100 years.”

Red lichen marks are now etched into many of the ancient relics at Persepolis. The spread of lichens, which dissolve minerals and penetrate stone surfaces by more than 1.5 centimetres, is driven by industrialisation, acid rain and the harsh desert climate, as per lichenologist – Mohammad Sohrabi. Iran is home to over 3,000 species of lichens, with 500 to 700 varieties growing on historical monuments. Many of Persepolis’s intricate motifs have already been lost due to lichen activity. Bisotun, another UNESCO World Heritage Site, featuring a massive carved inscription recounting the conquests of King Darius I, has also suffered much degradation due to lichen growth.

Combatting the lichen infestation at Persepolis has been ongoing in a painstaking process, where lichens are destroyed using modern techniques like lasers and substances that act like antibiotics which weaken the lichens enough, to be then removed with suction devices.