This extraordinary 60-minute guided 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 the tunnels are open to the public. Bring extra clothing – it's a chilly 10°C underground. Tours are given in English at 2.20pm daily from April to October; English-language audio guides are available at other times.
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.