The cotton castle is the hardened white calcium deposit from the hot springs.

I just went to the Yellowstone National Park less than two months before this trip, and saw the similar white Mammouth and the other beautiful hot spring areas. Everyone had to walk carefully on the boardwalk to avoid falling down. (Here's the link: https://pbase.com/denise_jtl/yellowstone )

At Pamukkale, the hot springs are different, as they're good for the skin. Tourists can wade freely in it, walking on the jelly-like mud from one spot to another.

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