Psychedelic salt mine

Hundreds of feet below a Russian city is an abandoned salt mine which might as well be the inside of a rave. The walls are covered with psychedelic patterns, caused by the natural layers of mineral carnallite creating swirls throughout the coloured rock. Carnallite is used in the process of plant fertilisation, and is most often yellow to white or reddish, but can sometimes be blue or even completely colourless. 

From the Daily Mail, via Joanne Casey's I Have Seen The Whole Of the Internet.  More from Wikipedia:

Carnallite is an evaporite mineral, a hydrated potassium magnesium chloride with formula KMgCl3·6(H2O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with rare pseudohexagonal orthorhombic crystals. The mineral is deliquescent (absorbs moisture from the surrounding air) and specimens must be stored in an airtight container... only forms under specific environmental conditions in an evaporating sea or sedimentary basin. It is mined for both potassium and magnesium...