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 date of the earliest catacombs in Odessa is difficult to determine (as they were all widened at a later date), but they likely date back to the 1600s, if not farther. However, the catacombs began to truly grow into their astonishing, labyrinthine form in the early 1800s, when the limestone quarried from them was used to build much of the city.
The Catacombs are on three levels and reach a depth of 60 meters below sea level. It is one of the world’s largest urban labyrinths, running up to 2,500 kilometers. 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 September 1995, in honor of the 200th anniversary of Odessa, the record for the longest underground journey in the catacombs was broken by a 27-hour continuous journey of over 40 kilometers. (Had they walked in a straight line the journey would only have been 9.5 kilometers.)
According to wikipedia; atlasobscura