Thames Barrier Visitor Centre, London

This construction cost a staggering £500 million, but it is an essential defense against the natural disaster of flooding. It has been used more than 20 times since it came into existence in 1982. A model of the barrier is in the Visitor Centre, as is much information pertaining to the purpose of the barrier, how it works and how it was built. The futuristic barriers bear an uncanny resemblance to a fragmented Sydney Opera House. Once a year in autumn, the barrier is tested, allowing you to see the steel gates raised at high tide. Call for information.