St. Michael's Cave or Old St. Michael's Cave is the name given to a network of limestone caves located within the Upper Rock of Gibraltar. In 1942, it was decided that an alternate entrance was required to improve air circulation within the emergency hospital which was located in the lower chambers of the cave, as well as to serve as an emergency exit in case of airstrike. Whilst blasting the rock in order to create this extra opening, another deeper system of caves known as New St. Michael's Cave (sometimes referred to as Lower St. Michael's Cave) were discovered. The series of descending chambers are riddled with examples of almost all known cave formations, including an underground lake of crystal clear water