Meteora is a complex of huge, dark-colored rocks of sandstone, rising out of Kalambaka, near the first hills of Pindos and Hassia. The Meteora monasteries, built on the top of some of the rocks, are now the second most important monastic complex in Greece, after Mount Athos. Of the thirty that have historically existed, only six are now operating, which since 1988 have been included in the UNESCO World Heritage List. The Christian ascetics benefited from the wild and inaccessible landscape, therefore they settled in the area. Meteora also due to their morphology, offered during the Ottoman domination an ideal refuge for monasticism and many monuments of post-Byzantine art have been preserved as well. In the early 19th century many monasteries were ravaged by Ali Pasha's army.