The "Great Bath" at the Roman Baths, Bath, Somerset.
The baths were built in the 1st century AD and are filled by a natural hot spring at 46C(115F). The Great Bath is lead-lined to prevent cold ground water from entering and cooling the bath.
There are notices around the baths that you must not touch the water.
I asked why we were forbidden to touch it when for 2000 years people had bathed in it.
They said that it was not treated and was dangerous!!!!
I touched it anyway.