Prague underground are more than 200 structures, most of them rather small. But in contrary to most other cities, there are four different kinds of underground structures: karst caves, pseudo-karst structures, mines, and subterranea. This is extraordinary as most cities only have subterranea.
The karst caves are frequent but rather small. Most of the caves are only a few meters long, the most important caves have been destroyed during the centuries. But the 32 caves are not so much of speleological interest, their sediments contain animal and human remains and are of great plaeontological and archaeological interest. However, none of them is of touristic interest. Some caves are even below ground water which is corresponding with the Vltava river. Obviously they were formed before the valley floor hat the current level.
Both Proterozoic and Cretaceous rocks at Prague contain pseudokarst caves. One of the most famous was discovered in 1883 during the operation of the quarry in the Šestákova Skála. A cavity in the Šárka lydites, 6 m long, 2 m high and 2 m wide, covered with quartz crystals up to 3.5 kg. Unfortunately this cavern was destroyed subsequently.
In the vicinity of Prague are deposits of coal and ores. At Prosek sandstone was quarried.
There are numerous subterranea all around Prague, as typical for any big city. The most important underground work in Prague, a technical monument of European importance, is the Rudolph Gallery. Beneath water conduits, there are sewer, water collectors, and cellars. The cellars below the castle have been converted into secret bunkers including an underground hospital during the cold war. They are not open to the public, although there was an event where visitors could see the hospital.