Discover rose-tinted valleys, towering ‘fairy chimneys’ and entire underground cities on a visit to this spectacular area of ...

At its peak, it could hold up to 20,000 people. The city spanned seven levels underground, with four main ventilation channels and over 50,000 other smaller shafts.

Credit: Youtube Helsinki’s underground city, built into solid bedrock, includes gyms, shops, and even a church. It’s designed to house 600,000 people in an emergency.