Bound by buildings on one side and a harbor on the other, the Finnish capital of Helsinki is pursuing a unique direction for its urban expansion: straight down. Thanks to a cooperatively shallow bedrock, building underground is relatively painless and inexpensive, and already citizens enjoy access to a subterranean swimming complex, shopping area, hockey rink, and more.
In some cases, it makes even more sense to stick a facility underground than it does to have it up above: a data center built beneath a cathedral uses cold sea water to cool its machines, drastically cutting energy consumption, and a subterranean coal storage facility tucks unsightly silos conveniently out of sight. Helsinki is actively working more underground developments, most of which are connected by a sophisticated network of tunnels. There's plenty of space underground, the mayor explains. "Why not use it."