A 20-mile maze under Budapest hid a secret WWII aircraft engine plant

The Kőbánya cellar system under the 10th district of Budapest is, according to Wikipedia, "the largest cellar complex in the country" — a network of tunnels estimated at 32 to 35 km long beneath 44 to 54 acres. It began in the Middle Ages as an underground limestone quarry that supplied stone for the Hungarian Parliament, the Széchenyi Chain Bridge, and the State Opera House. — Read the rest

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