Built during the Second World War as an air-raid shelter for the citizens of the Upper Town, the Grič Tunnel runs about 350 metres beneath the Gradec hill, with six entrances opening onto different streets. After the war it was forgotten — locked, mostly used as a damp shortcut by people who knew the right doors — and only reopened to the public in 2016.
It is now free to walk through, with permanent and temporary exhibitions along its length. The most popular is the immersive Time Machine exhibit, which tells the history of Zagreb through illuminated installations. The tunnel was also a famous venue for raves in the 1990s, when it briefly hosted the city's underground (literally) club scene.