It's not illegal to enter Odessa's catacombs, though it's not exactly encouraged, either. Every so often, someone disappears in the darkness for good. The almost 2,500-kilometres-long catacombs, laid out in a straight line, the labyrinth would stretch five times longer than the world's second largest catacombs underneath Paris (to get a sense of how long tunnel system that really is, it is only 2138 kilometers from Odessa to Paris).
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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 were greatly expanded in 1794, when Catherine the Great sought limestone to build her city by the sea. From that time on, they began to truly grow into their astonishing, labyrinthian form in the early 1800s when the limestone quarried from them was used to build much of the city.
Odessa's catacombs quickly became the preferred hideout of rebels, criminals, and eccentrics. During WWII although the Soviets had been forced out of the city they left behind dozens of soviet-organized Ukrainian rebel groups hidden below the city in the expansive catacombs.
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Every corridor and every room down there are different, with a variety of drawings on the walls: simple symbols carved out years ago or pictures of nude women coexist with the sketches of gangsters hiding from the czar in the late 19th century, Bolsheviks a decade after them and partisans hiding from the Nazis.
Today there is an entire Ukrainian subculture of catacomb explorers with dozens of semiprofessional groups, often quite competitive, exploring the catacombs. They go on multi-day underground treks, known as expeditions, to document and map the system. Should someone get lost in the catacombs, (as happens every couple of years) these groups put aside their differences and mount large search expeditions. They have rescued a number of children who have wandered into the catacombs.
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Only one small portion of the catacombs is open to the public, within the Museum of Partisan Glory in Nerubayskoye, on the north from Odessa. It would be extremely dangerous to try to explore the catacombs on one's own. However, professionals can be hired at the cost of approximately UAH 250 per person, depending on the time spent underground, the complexity and length of the route, the number of tourists and options for delivery to the site entrance to the underground area.