One of the strangest swimming pools in the world: a thermal bath inside the Hierapolis site where a 7th-century earthquake knocked a colonnade of Roman columns into the warm springs that fed the city's baths. The columns settled where they fell, and the pool has been used for bathing over submerged ancient marble ever since. The water is around 35°C, gently carbonated, and clear enough to see the column drums and Corinthian capitals lying on the bottom two metres below.
An additional entrance fee applies beyond the main site ticket, plus charges for lockers and towels. This is unavoidably touristic — bathing suits, sun loungers, the whole apparatus — but the experience of floating in warm mineral water while resting your feet on a genuine Roman column base is genuinely peculiar and worth doing. Avoid midday when tour groups arrive in force; morning and late afternoon are significantly calmer.