Odessa catacombs are the world’s longest network of extremely tangled labyrinths, galleries and tunnels, which is located under the city and its surroundings. These multilevel intricacies of underground tunnels stretch for 2.5 thousand kilometers and represent a real “parallel world”, many of which are unknown even to the cavers. Here, where the temperature prevails all the year approximately in +14 C, narrow manholes lead to huge halls, and wide corridors unexpectedly end in dead ends. In some parts of the catacombs groundwater come to the surface and form whole reservoirs.