This extraordinary tour explores the passageways below the old city. The earliest were probably dug in the 14th century, perhaps for beer production or defence; the latest date from the 19th century. Of an estimated 11km that have been excavated, some 500m of tunnels are open to the public. Bring extra clothing (it's a chilly 10°C underground).
Plzeň's wealthier set used to have wells in their cellars. Overuse led to severe water shortages. When wells dried up they were often filled with rubbish and buried; these have yielded an amazing trove of artefacts. English-language audio guides are available in case you miss the English tour.