Barcelona

Maps, plans and photographs are used to explain the history of Barcelona's sewerage system, and city sewerage systems in general, from the Roman Empire to the present day in this unusual museum. It's part of a private foundation belonging to Catalunya's Institute of Engineers. You enter the building through a glass booth similar to a car park entrance, and once inside you'll see the cisterns and underground passages on which the health and well-being of the population largely depend. It's a unique experience and comes with a couple of drawbacks: the smell and the damp.

• Admission Charge: Free