Herzog & De Meuron have designed the Naturbad Riehen in Switzerland, a public bath house for visitors. While many public baths look very man-made this one takes on a natural look, and does not use traditional chemicals found in most baths.
The design has natural vegetation around the outside of the pool, adding to its non-constructed look. The pool also uses a biological filter process, so that although it looks like a pond, its cleanliness is up to regular standards. Having visitors come and swim in an actual pond would defeat the purpose of a bath house, so hygiene of the design is still extremely important. Apparently the pools can hold up to a couple thousand guests throughout the course of the day and still retain cleanliness.
Photo Credits: designboom, herzogdemeuron