The Odessa Catacombs are a labyrinth-like 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.
The Catacombs are on three levels and reach a depth of 60 metres (200 ft) below sea level. It is one of the world’s largest urban labyrinths, running up to 2,500 kilometres (1,600 mi). Parts were used as air-raid shelters during World War II. Part of the tunnels, only under the city, were turned into bomb shelters in the Cold War. Such bomb shelters supposed to be refuge for civilians in case of nuclear strike or gas attack.
In the 19th century, most houses in Odessa were built of limestone that was mined nearby. These mines were abandoned and later used, and widened, by local smugglers, creating a labyrinth of tunnels beneath Odessa. Stories of smugglers are part of urban legends about treasures hidden underground. Despite fairly plausible details, such stories can hardly be proved. Many of the tunnels under living areas were filled up with earth, concrete or sand by construction companies, and are no longer available.