I'm a big fan of what I consider to be engineering marvels of the world. Big public works projects that may or may not no economic sense in a pure cost benefit way but add to the economy and a wonder to see. The ones that make me step back and go "Wow - how did they do that!?". Hoover Dam, Panama Canal, big bridges and the Thames Flood Barrier (to name a few).
The Thames Flood Barrier protects London from flooding. I really want to know how they did it. Unfortunately, while I've taken a sightseeing cruise out a few times I've never made it to the information center. And it doesn't seem to be a big sightseeing landmark.
More information on Wiki: http://en.wikipedia.org/wiki/Thames_Barrier