What Lies Beneath?

Now here is an idea for a story setting! I was reading my English Heritage magazine and came across a reference to the medieval vaulted passageways that lie beneath the City of Exeter. Apparently they were built to supply fresh drinking water to the city and are unique in the UK. These days they are a tourist attraction.

Exeter's early water supply came from springs and the Roman garrison sourced water from two local springs and brought it into the city via an aqueduct. In the Middle Ages...