The rock formations of Pamukkale (literally, "Cotton Castle") are a series of white terraces saturated with dissolved calcium bicarbonate, bubbling up from the feet of the mountains beyond. As the water flows over the edge of the plateau and cools, carbon dioxide is given off and calcium carbonate precipitates as hard chalk, or travertine. The spring emerges in the middle of the ancient resort city of Hierapolis (the ruins themselves are worthy of a stop as well). Sadly, the travertine that makes up the Pamukkale is gradually turning from white to yellow, and is even thought to be drying up. Thirsty hotels are blamed by the government, which has demolished many of them. This is good for the Pamukkale, but it makes finding a hotel increasingly difficult. Most stay in Pamukkale Koyu. The Hierapolis archaeological site lies west of Pamukkale Koyu. Its main features include a temple of Apollo and the adjacent Plutonium, a cavern emitting a deadly sulphur and carbon dioxide gas. East of here is a restored Roman ampitheatre, dating from the second century AD. Most of the stage buildings and their elaborate reliefs remain intact. An interesting part of the city is the colonnaded street, once extending almost 1km to a series of monumental portals. Today, only the most northerly, a triple arch flanked by towers and dedicated to the Emperor Domitian in 84 AD, still stands. Just south of the arch is the elaborate tomb of Flavius Zeuxis. This is only the first of more than a thousand tombs that constitute the full necropolis, the largest in Asia Minor.