Explores how Sweden has gone to extraordinary lengths to protect itself - by excavating 65,000 nuclear bunkers. Plus, Somerset's Cheddar Gorge and an underground first in Romania.

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An underground city beneath a forest in Northern France built to house 3,000 people, an ice cave under Iceland's biggest glacier, and a subterranean power station in London.

British documentary series exploring incredible feats of human endeavour underground, revealing what people have built, where they have built it, how and why.

British documentary series exploring incredible feats of human endeavour underground, revealing what people have built, where they have built it, how and why.

British documentary series exploring incredible feats of human endeavour underground, revealing what people have built, where they have built it, how and why.

British documentary series exploring incredible feats of human endeavour underground, revealing what people have built, where they have built it, how and why.