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Turkey's underground city of 20,000 people
The Hamas tunnels: a wildcard in the Gaza fighting
Malta's secret tunnels: inside the newly discovered underworld of Valletta, Cities
Presidio Tunnel Tops: Infrastructure Designed for 360 Views and Fun – THE DIRT
What are the limits of how long and deep an underwater tunnel can be built to be usable for railway and road traffic? - Quora
floating tunnels
You May Not Know It, But You're Living In The Golden Age Of Tunneling
The incredible tunnels pushing deep below the surface
The World's 20 Most Impressive Tunnels
The Great Underground
Wind' from Earth's middle layer blows through a secret passage beneath Panama
London Underground: Longest journey on the Tube in just one tunnel where you won't see sunlight for miles - MyLondon