Moonmilk

Moonmilk (sometimes called mondmilch, erroneously (influenced by "mountain") montmilch or rarely mundmilch) is a white, cheese-like substance found inside caves. It is similar to other deposits, but its unique quality is that it does not harden or turn to stone. It is a precipitate from limestone comprising aggregates of fine crystals of varying composition usually made of carbonate materials, e.g., calcite, hydromagnesite, and gypsum.

Hypotheses about the origin of moonmilk are mixed. Some scientists think it is a bacterial action rather than a chemical one. In this theory, it is thought to be created by the bacterium Macromonas bipunctata. Certain bacterial actions are known to be capable of breaking down stone to form this semiliquid "milk".

It was originally thought (by Conrad Gesner, 1555) to be created by "moon rays."