What do you do when your city has no room to grow, hemmed in by existing construction on one side and the sea on the other? If you’re New York City, you build up. If you’re Helsinki, you build down. Down as in a few hundred meters down (direct link).
Thanks to shallow bedrock and a quiet seismic environment, Helsinki has put together an underground master plan and is moving all sorts of infrastructure facilities into its expanding maze of tunnels. This frees up surface real estate for more attractive uses.