But it takes hundreds of thousands of pounds of the stuff to hold together even a small city--most of it laboriously buried beneath the streets.

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The flood alleviation scheme consisted of two parts - work to repair, replace and upsize 600m (660 yards) of culverts beneath the streets of Penrith.

BBC: Flood alleviation scheme complete in Penrith

Then, inspired by NYC's High Line, Fletcher Priest won the Green Infrastructure Ideas Competition with his proposal for an underground mushroom tunnel beneath the streets of London.

ENGADGET: Inhabitat's Week in Green: Bicymple, computer-age fossils and an underground mushroom tunnel

Beneath the city streets that travellers walk on each day, dark labyrinths of underground catacombs are passageways to the past, to a time when the ghostly tunnels served as burial grounds for millions of people.

BBC: Exploring the history of catacombs

Beneath the veneer of its squeaky-clean streets, super-efficient tram systems, rigorous banking principles and holier-than-thou Alpine churches lies a somewhat stranger city that is ready to surprise at every turn.

BBC: The stranger side of Switzerland

Markets bustle with people, dogs, and pigs, impromptu riots form in the streets and your character ducks beneath hanging laundry before hiding in a covered wagon.

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The rest walk from downtown to the neighborhoods, scattering in all directions across bridges, through underpasses, beneath trestles, up hills to dark residential streets, solitary figures beneath thought bubbles of warm breath and cigarette smoke.

NPR: Excerpt: 'Citizen Vince'