A relic from the Spanish Civil War hides in this picturesque Spanish park. There, among the postcard-perfect statues and greenery, is an old bunker buried beneath a thick layer of dirt and grass.

Known as Posición Jaca, the bunker was used by the Republican forces to defend Madrid from Nationalists. If attacked, high-ranking Republicans could seek refuge in the secure subterranean space. Covered beneath feet of soil and confined within the sturdy walls, they’d remain safe from bombs beating the surface above them.

Two hundred people could spend months hunkered down in the bunker, which was surprisingly spacious and equipped with toilets, offices, and electricity. Chimneys poking out of the earth provided ventilation and air circulation, and a communications system would keep the refugees in touch with the outer world.