Thames Barrier in London. If you've never seen it in person, it is a mammoth piece of engineering. Its genesis lies in a huge flood that hit the North Sea in 1953, killing hundreds of people. At the time, flood barriers weren't particularly common, and the Thames Barrier was so big that it took around thirty years to be built. But we're all entering an age now in which climate change and associated sea level rises have become more of a concern, and the idea of flood protection is soaring up the agenda.
I didn't know this until recently, but New York is amongst the most at-risk cities in the world. And of course nothing highlighted this fact more than last year's devastating Hurricane Sandy. Now, unsurprisingly, scientists and politicians there are concerned that if they don't build a barrier or some other kind of protection around the city soon, parts of it may not survive the next century. About a month ago, I travelled to New York to meet the people working to solve this problem.... and my journalistic gatherings from there have now been turned into a documentary for the BBC World Service, which you can hear this week on radio, or online.