Turkey Has a Whole Hillside of White Stone Bathtubs

From the road across the plain it looks as though somebody left the laundry out. A whole hillside in south-west Turkey, nearly two hundred metres of it, is white — not snow-white in patches but white all over, in folds and ledges and frozen ripples, with pools of pale blue water sitting in the ledges like spilled sky. The Turkish name for it is Pamukkale, the cotton castle, and from a distance that is very nearly a literal description.

Up close the white turns out to be stone, and the stone turns out to be new. Seventeen hot springs rise at the top of the cliff, the mildest of them about as warm as a comfortable bath at 35 degrees and the hottest arriving at a full boil. Their water is led some three hundred metres to the head of the terraces and then let go, and as it spreads and runs downhill it lays a thin skin of white mineral over everything it touches. Do that for a few thousand years and you get a hillside.

What it builds is a staircase of shallow basins, each one rimmed by a raised white lip that the water poured for itself. A full basin spills into the one below, which spills into the next, all the way down. In between, the same process has made what the World Heritage listing calls mineral forests and petrified waterfalls: curtains of stone hanging off ledges, ripples set solid in mid-ripple. The terraced ground runs something like two and a half kilometres along the hill.

People have been coming to sit in it for a very long time. At the top of the cliff stand the ruins of Hierapolis, a spa town founded at the end of the second century BC by the kings of Pergamon precisely because of the hot water, with baths, temples and a theatre added over the centuries that followed. The springs had a working life too: the hot mineral water was good for scouring wool and for drying it. A listing in 1988 put the white hillside and the town above it on the World Heritage list as one site.

The reason the water builds things instead of dissolving them is a matter of pressure and fizz. Underground, hot water under pressure dissolves limestone happily, picking up calcium and carbon dioxide until it carries far more than it could ever hold at the surface. The instant it emerges the pressure drops and the carbon dioxide begins escaping into the air, exactly as it does from an opened bottle of sparkling water. Losing the gas tips a chemical balance, the dissolved calcium can no longer stay dissolved, and it comes out as calcium carbonate.

Freshly laid, it is not stone at all. It is a soft white gel that a fingertip would mark. Left alone it crystallises and hardens into travertine, the same stone quarried elsewhere for floors and building fronts, which makes Pamukkale a quarry that is still running and producing roughly as fast as anyone can stand there and watch.

If that sounds exotic, the same thing is going on in your kitchen. The chalky crust inside a kettle in a hard-water area is this reaction run at speed: heat drives the carbon dioxide out of the water, the dissolved calcium gives up, and a layer of calcium carbonate is left on the element. Pamukkale is limescale on a geological budget, with a much better view.

Being new stone, it is also soft stone, and it needs looking after. The flow is rationed between sections on a rota now, and stretches are closed in turn so the white can grow back over ground that has gone grey. Visitors walk one marked route, barefoot, through warm water a few centimetres deep. That seems a small price for being allowed to stand on a hillside that is still under construction.