Today, underground cities are more about escaping the weather than escaping an invading army. Modern subterranean networks are booming in extreme climates.
Finland’s capital is actively building "down" instead of "out" to curb urban sprawl. Their underground city includes everything from swimming pools and a hockey rink to the world’s greenest data center, cooled by the sea. Underground Cities
Subterranean structures offer natural insulation , reducing the energy needed for heating and cooling. Today, underground cities are more about escaping the