From CHN:

The northern staircase of Apadana Palace in Persepolis , depicted by designs of gift givers has been hidden under lichens during 2500 years old. In an attempt to rescue this world heritage site, Iran with support of UNESCO has announced an international call for cooperating in cleaning up the lichens with help of domestic and foreign restoration experts.

Announcing this news, Mohammad Hassan Talebian, head of Parse Pasargadae Research Center, told CHN: “Northern staircase of Apadana Palace is one of the most important sections of Persepoise World Heritage sites. Since it was situated unprotected in free space for over 2500 years and endured harsh weather and rain and taking into account that it was out of direct sunlight, it flecked with lichens over time. Cleaning up lichens would be a time-taking job and we have started the project with the assistance of Iranian and foreign experts.”

Pointing that lichen is a real threat to a large number of stone historical sites in the world and have posed serious harms to some of them, Talebian added: “Some approaches have been undertaken to fight this blight, although they were not always useful. This time we are hoping to be able to treat these apparently incurable lichens from Persepolis body.”

According to Hassan Rahsaz, stone restoration expert in Persepolis, fortunately there is no worry about the other parts of Persepolis from this regard. He further said that Iranians have also a long experience in cleaning up lichens and this time is a hope to overcome this problem completely through international cooperation.

Founded by Darius I in 518 BC, Persepolis was the capital of the Achaemenid Empire (550-330 BC) which is situated in Iranian Fars province. It was built on an immense half-artificial, half-natural terrace, where the king of kings created an impressive palace complex inspired by Mesopotamian models. Persepolis was devastated in 330 during the attack of Alexander the Great to Persia who set fire to the capital of Achaemenid Empire and leveled this huge monument to the ground. The importance and quality of the monumental ruins made it a unique archeological site. UNESCO declared Persepolis complex a World Heritage Site in 1979. Persepolis is also one of the 80 treasures featured on Around the World in 80 Treasures presented by Dan Cruickshank.