Moonmilk, also known as mondmilch or montmilch, comes from limestone and dolomite caves. There are various theories about its origin. Some scientists think it's the result of a chemical reaction, but others think it comes from the bacterium Macromonas bipunctata, although studies haven't confirmed this.
The name comes from the belief that moonmilk was created by moon rays. Although that origin hypothesis isn't true, it's my favorite because it sounds so tasty.
Moonmilk was used in both prehistoric cave art and as a medicine in the 16th century. People claimed that it cured acidosis and had a neutralizing effect on acid. According to wikipedia, the substance has no adverse health effects, but I wouldn't risk trying it… especially after hearing about the bacteria.