St Michael's Cave is the name given to a network of limestone caves located in the Upper Rock Nature Reserve of Gibraltar, at a height of over 300 metres above sea level. Its name is derived from a similar grotto in Monte Gargano near the Sanctuary of Monte Sant'Angelo in Apulia, Italy, where the archangel Michael is said to have appeared.

It is the most visited of the more than 150 caves found inside the Rock of Gibraltar, receiving almost 1,000,000 visitors a year.

The cave was created by rainwater slowly seeping through the limestone rock, turning into a weak carbonic acid which gradually dissolved the rock. Through this process, tiny cracks in The Rock's geological fault grew into long passages and large caverns over thousands of years. The numerous stalactites and stalagmites in the cave are formed by an accumulation of traces of dissolved rock deposited by water dripping from the ground above.

http://en.wikipedia.org/wiki/St._Michael%27s_Cave