Famous underground systems of Odessa are often visited by diggers who even invite other people to join them. The place is often examined by tourists from other cities and countries.
The tunnels of the system are formed by a great variety of old stone-pits in Odessa that stretch to 2.5 thousand km.
Many call the tunnels 'catacombs' which is not correct as catacombs are represented by ground disposal whereas here caves, basements, and bomb shelters are presented.
They started develop limestone in the beginning of the 19th century but later the production was forbidden due to frequent collapse and erosion of soil.