The Cathedral Cave was long believed to be bottomless, probably giving birth to the story that Gibraltar was linked to Africa by a subterranean passage over 15 miles (24 Km) long under the Strait of Gibraltar. The cave consists of an Upper Hall with 5 connecting passages and rocks between 40ft (12.2m) and 150ft (45.7m) to a smaller hall. Beyond this, a series of narrow halls leads to a further succession of chambers, reaching depths of some 250ft (62.5m) below the entrance. During World War II the cave was prepared as an emergency hospital, but was never used as such.

St. Michaels Cave now used as an entertainments venue

Whilst blasting an alternative entrance to the cave, a further series of deeply descending chambers ending in a mini lake were discovered and named Lower St. Michael's Cave. The Cathedral Cave is opened to visitors and makes a unique auditorium for concerts, ballets, drama and presentations. The unique beauty of crystalised nature can be appreciated through a centuries old stalagmite that was too heavy and fell on its side at the far end of the chamber.

In 1972 a slice 18" thick (45cm) was cut, revealing its interior structure. Its growth is clearly indicated during periods of excessive rain by light brown rings and patches, the dark area being formed during periods of less rain. Two thin lines of a crumbly white substance are thought to represent glacial periods. The stalagmite is also translucent in some areas.