Apparently since the medieval times Plzen had an elaborate set of underground passages, sometimes…read moreforming three overlapping levels in depth. The passages were dug mostly for storage, although water control and siege protection also played some part. A small part of these passages has been restored, even smaller part is currently accessible for organized tours with a tour guide, departing approximately once per hour. Tours are mostly in Czech; these can be used by non-Czech-speakers via the use of audioguides; few time a day the tours are held in English, German, and Russian. The cost of the tour is approximately $10, this includes a coupon for free (small) beer in the nearby restaurant.
Expect walking for about 1 km in a hardhat provided by the tour, in temperatures reaching 55F, on occasionally wet - but not flooded - floors. The tunnels themselves are quite interesting due to their elaborate structure; the museum placed some artifacts into the niches previously used for the storage, so one would also learn a bit about the Plzen's history.
Definitely worth visiting if you are in Plzen for some other reason.