Pamukkale

A white hillside 2,700 metres long, 600 wide and 160 high, built out of terraced pools that look like poured cotton, which is what the Turkish name means: cotton castle. Seventeen hot springs between 35 and 100 C carry dissolved calcium carbonate 320 metres down the slope, and where the water reaches air the carbon dioxide comes out of it and the carbonate settles as a soft gel that hardens into travertine. On top sits Hierapolis, a Greek city founded as a thermal spa in the second century BC that became a healing centre and later a Christian site where the Apostle Philip spent his last years. UNESCO listed both together in 1988. Over two million visitors a year, so the water flow is managed and sections are closed in rotation to let the terraces regrow. Barefoot only on the parts that are open. Denizli Province, Turkey.