Helsinki first began digging up its surface in the 1980s during the Cold War, with the aim of creating a subterranean space that could contain the city’s entire population in case of an attack.

https://www.helsinkitimes.fi/world-int/world-news/finland-in-the-world-press/20081-underground-city-gambling-restrictions-and-largest-hotel-finland-in-the-world-press.html