Naturally terraced pools are a shallow, rimmed water bodies usually located on the travertine terraces - a form of limestone deposited by mineral springs, especially hot springs. Travertine terraces often have a fibrous or concentric appearance and exists in white, tan, and cream-colored varieties.
Because of this and because of the biochemical composition of the water, these terraced pools usually have wonderful shades of different colors.
(thanks Bosko)