The white stone is called travertine. It is made by warm water that bubbles up out of the ground from deep underground. The water is full of a mineral called calcium carbonate (the same thing that makes up chalk and seashells). When the water cools at the surface, the mineral hardens into snow-white rock.
Over thousands of years, the water has dripped down the hillside layer by layer. The rock has built up into bowl-shaped terraces, one stacked above another. Each one looks a bit like a small pond with smooth white edges, holding warm water in the sunshine.
The water arrives at the top warm, because it has been heated by the rocks deep underground. It is around 35掳C - about the same as a comfortable bath. The Romans built a town here 2,000 years ago just so they could come and bathe in the warm pools, and you can still see the ruins of the old Roman streets next door.
Today Pamukkale is so precious that visitors have to take their shoes off to walk on the white terraces. That keeps the rock clean and white, and protects the slow drip-drip-drip that has been building this place for thousands of years.