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.
Wineries and beer breweries later moved into the abandoned sections, and the Dreher brewery still uses some of the cellars. During the Second World War, "the dimensions of the complex enabled it to be used as a covert aircraft engine assembly plant and a civilian hideout." Workers built engines for the Messerschmitt Me 210 underground and "were able to produce more than 200 engines until production was relocated into Germany as Red Army troops came close to Budapest."
The limestone keeps the spaces "moist and moldy," and parts are flooded by groundwater. In 2004 a 62-year-old man fell into a pit that opened in a park built over filled-in tunnels; according to Wikipedia, "the victim died because of the toxic gases of the rotting waste, not from the fall."
