Beginning in the 1700s, people have mined limestone from the catacombs underneath the city of Odessa in Ukraine. Over time, this catacomb system became the largest in the world. Exploration groups map an estimated 30 kilometers of mysterious tunnel every year — at this rate, it will take them 30 years to map the entire system. The total length of the system covers more distance than that between Odessa and Paris.

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Originally published on Daily Curiosity