Odessa Catacombs Wikipedia
The Odessa Catacombs are a labyrinthlike network of tunnels (subterranean cavities) located under the city of Odessa and its outskirts in Ukraine, that are mostly (over 90%) the result of stone mining, particularly coquina. The system of Odessa Catacombs consists of a network of basements, bunkers, drainage tunnels and storm drains as well as natural caves.