The largest catacomb system in the world still remains unmapped.  It is located in Odessa Ukraine, and aptly called The Odessa Catacombs. They are a labyrynth of man-made tunnels that are situated underneath Odessa.  There are over 1,500 miles of catacombs that are carved into the limestone beneith the city.  Unlike the catacombs of other eastern cities, the Odessa Catacombs were never used to bury the dead.  It is thought the catacombs were formed from mining the limestone that was used to construct houses in 0dessa.  The catacombs date back to the 17th century and were believed to have expanded during the late 18th century.    Mining continued until the 20th century. The mines later became abandoned and they were most likely widened by smugglers into a labyrinth of underground tunnels.  The catacombs are also considered to be the oldest official underground museum.  Only a small portion of the catacombs is open to the public. They contain hidden caves, hideouts, and old artifacts left over from smugglers, criminals and WW II Soldiers.