A number of cities (Paris and Budapest amongst others) are built above the stone quarries that supplied the raw material for many of their above-ground structures. Perhaps the most impressive example however is Odessa, beneath which lies a claimed 2,500 kilometres of former limestone quarries (underground workings for building stone are by tradition known as quarries rather than mines).
The quarries are almost universally known as catacombs but their function was rarely if ever religious. The subterranean network extends beyond the city itself; in central areas a proprtion of the quarries have now been back-filled for stability.
Most of the network was excavated in the 19th century and in places the quarries extend to three levels and a depth of over 50 metres.. There is a smaller network of natural caves, in places intecepted by the workings. Early use of the tunnels after the quarrying had finished included storage and smuggling, although the extent of the latter use is as usual often embellished for tourists.
In World War II, many parts of the so-called catacombs were used as air-raid shelters. Sections were also used to conceal Soviet Partisans who fought against the Romanian and German invaders during Operation Barbarossa. In the Cold War, some fall-out shelters were also built.
Various parts of the network can be visited although not all guides are licensed. There also some museums which tell the story of the catacombs. There are numerous tales of explorers becoming lost and perishing but many of these lack any firm evidence and are at least part urban myth.