Many cities have extensive underground layers for various reasons. These subterranean spaces often begin with separate basements and tunnels that eventually become connected to each other, or possibly the area was once above ground, and was just built over. The capital of Finland is a different story. Helsinki built its underground city in one fell swoop. Underneath the ground level streets, you'll find shopping centers, a church, a museum, sports venues, and more.
Thought to be the world’s only city with an underground master plan, Helsinki began excavating tunnels through bedrock in the 1960s to house power lines, sewers and other utilities. City planners quickly realized that the space could also be home to retail, cultural, and sporting attractions—and that it could shelter the city’s population of 630,000 in the event of another invasion from the East. The building of the tunnels expanded with new purpose.
Tomi Rask, a preparedness instructor for the city of Helsinki’s rescue department, says the alternative purpose of the tunnels is to “save people against the actions of war.” No Finnish government official would ever mention Russia as the reason for such defensive preparations, but they don’t have to.
Read about both sides of Helsinki's underground at Atlas Obscura.