Exeter's Underground Passages, which once served as conduits for fresh water, are the only medieval vaulted passages open to the public in Britain. They date to the mid-14th century, although some were enlarged by the Victorians. An exhibition and video precede the 25-minute guided tour. Many of the passages are narrow and low: be prepared to stoop. The tours often sell out during school vacations, so come early. Children under five are not permitted in the tunnels.
2 Paris St., Exeter, EX1 1GA, England