The so-called manual-block system takes the place of a dozen separate signals that help drivers adjust their speed and avoid colliding with other trains in the dark tunnels.

WSJ: PATH's Road Back Is Rocky

The numerous grape varieties are grown elsewhere in Austria, but maturation of the sparkling wines takes place in the dark 2.3km network of stone and brick tunnels that run under the northern 19th district.

BBC: In the kitchen with Viennese history

This spider's web of narrow tunnels, complete with coffin-sized cavities, opens out into dark subterranean churches - a most evocative way to end an underground tour of Rome.

BBC: Going underground at the Colosseum

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