If you've seen Trainspotting, you'll likely think Edinburgh's underworld is a tourist no-go. But the good burghers of Auld Reekie have a history of the macabre that goes back to long before Irvine Welsh's lovable junkies ran amok in the city: Witches, hangings, plagues, and ghosts are all just part of the fabric. Today this darker side of adventure tourism is getting a fresh airing with the opening of more sections of Edinburgh's underground city, known as the vaults or catacombs.

The brutal machinations of Edinburgh's medieval history are literally buried beneath the surface. In the 15th and 16th centuries, Edinburgh's population rapidly increased, and with it the slums that crowded the volcanic mound around the castle. Here, a labyrinth of squalid and disease-filled alleyways spread deeper, eventually leading to a devastating outbreak of the plague. Hardly the most compassionate of solutions, city officials closed off entire sections of the slums to contain the Black Death, condemning sick (and perhaps other still healthy) residents to gruesome deaths. Consequently, the vaults beneath Mary King's Close, one of these quarantined areas, are said to be one of "Britain's most haunted locations."

The Real Mary King's Close (2 Warriston's Close; +44.8702.430.160, www.realmarykingsclose.com), jointly operated by the Edinburgh City Council and a private consortium, leads visitors into the old warren of tunnels and unveils those squalid 16th-century living conditions—a historical perspective on the fate of the slum dwellers during the plague. One visiting psychic, it is said, felt a young child tug at her pant leg as she tried to leave. Perhaps that child was Annie, a young ghost said to wander in rags through her underground tomb, lamenting the loss of her plague-stricken family. Feel the paranormal for yourself and decide whether you agree with the psychic, who described Mary King's Close as "the unhappiest place ever."