Pamukkale (Cotton Castle), Denizli Province, Southwestern Turkey, turkey

Pamukkale, meaning "cotton castle" in Turkish, is a natural site in Denizli Province in southwestern Turkey. The area is famous for a carbonate mineral left by the flowing of thermal spring water. It is located in Turkey's Inner Aegean region, in the River Menderes valley, which has a temperate climate for most of the year.

The ancient Greco-Roman city of Hierapolis was built on top of the travertine formation which is in total about 2,700 metres (8,860 ft) long, 600 m (1,970 ft) wide and 160 m (525 ft) high. It can be seen from the hills on the opposite side of the valley in the town of Denizli, 20 km away. Known as Pamukkale (Cotton Castle) or ancient Hierapolis (Holy City), this area has been drawing visitors to its thermal springs since the time of Classical antiquity. The Turkish name refers to the surface of the shimmering, snow-white limestone, shaped over millennia by calcite-rich springs. Dripping slowly down the mountainside, mineral-rich waters collect in and cascade down the mineral terraces, into pools below. Legend has it that the formations are solidified cotton (the area's principal crop) that giants left out to dry.

People have visited area for thousands of years, due to the attraction of the thermal pools. As recently as the mid-20th century, hotels were built over the ruins of Hierapolis, causing considerable damage. An approach road was built from the valley over the terraces, and motor bikes were allowed to go up and down the slopes. When the area was declared a UNESCO World Heritage Site, the hotels were demolished and the road removed and replaced with artificial pools.[citation needed] There are well-preserved Roman ruins and a museum on site. A small footpath runs up the mountain face for visitors to use, however the travertine terraces are all off-limits, having suffered damage, erosion and water pollution due to tourism. 

Pamukkale's terraces are made of travertine, a sedimentary rock deposited by mineral water from the hot springs. In this area, there are 17 hot springs with temperatures ranging from 35 °C (95 °F) to 100 °C (212 °F). The water that emerges from the spring is transported 320 metres (1,050 ft) to the head of the travertine terraces and deposits calcium carbonate on a section 60 to 70 metres (200 to 230 ft) long covering an expanse of 24 metres (79 ft) to 30 metres (98 ft). When the water, supersaturated with calcium carbonate, reaches the surface, carbon dioxide de-gasses from it, and calcium carbonate is deposited. Calcium carbonate is deposited by the water as a soft gel which eventually crystallizes into travertine.

Pamukkale is recognized as a World Heritage Site in 1988. It is a tourist attraction due to this status and its natural beauty.

The underground volcanic activity which causes the hot springs also forced carbon dioxide into a cave, which was called the Plutonium, which here means "place of the god Pluto". This cave was used for religious purposes by priests of Cybele, who found ways to appear immune to the suffocating gas. The hotels built in the 1960s were demolished as they were draining the thermal waters into their swimming pools and caused damage to the terraces. The water supply to the hotels is restricted in an effort to preserve the overall site and to allow deposits to regenerate. Access to the terraces is not allowed and visitors are asked to follow the pathway only.