Deep in the earth beneath Pamukkale and the ancient Roman city of Hierapolis lies a vast source of water heated by volcanic lava. The water dissolves pure white calcium, becomes saturated with it, and carries it to the earth's surface, where it bursts forth and runs down a steep hillside.
Cooling in the open air, the calcium precipitates from the water, adheres to the soil, and forms white calcium "cascades" frozen in stone called travertines.
The water has been bursting forth at Hierapolis/Pamukkale for more than two millennia. The Romans built the spa city of Hierapolis so citizens could come and enjoy the health benefits of the hot mineral water. The beauty of the travertines was just a bonus.
Pamukkale, which has been used as a spa since the second century BC, literally means "cotton castle" in Turkish.
The travertine features have their origins in the shifting of a fault in the valley of the Menderes river (between here and Denizli). As the fault shifted, very hot springs with a very high mineral content (notably chalk) arose at this location. Apart from the slightly radioactive minerals, the calcium and hydrogen carbonate react to create calcium carbonate (also known as travertine) and limestone. This is what gives Pamukkale its whiteness and created the pools.
Pamukkale is in the Unesco's World Heritage list.