Beneath Budapest lies a vast underground network—a twenty-mile maze of limestone tunnels carved over centuries. According to Boing Boing, the Kőbánya cellar system began as a medieval quarry supplying stone for Hungary's grandest buildings: the Parliament, the Széchenyi Chain Bridge, and the State Opera House. But during World War II, these ancient caverns took on a secret purpose: they became a hidden aircraft engine manufacturing plant, shielded from bombs by thousands of tons of limestone. Today, the labyrinth stands as a remarkable testament to both medieval engineering skill and wartime ingenuity.

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