The Thames Barrier (above) is an impressive and strangely beautiful structure. It is the second largest moveable flood barrier, with rotation mechanisms contained within the stainless steel clad piers which remind me a little of the Sydney Opera House. There are ten spans in total: the four main 61 metre spans were designed to be the same width as that at Tower Bridge.
London depends upon it raising the gates when high tide and a storm surge combine to send water funnelling up the Thames estuary, threatening to flood the capital. And the risk is becoming ever greater, as the south east of England is gradually sinking while water levels rise higher.
And this is another view of the Barrier, using sound not light:
The photo was taken just after we powered through one of the gaps, and you can see the flat bit which shows the gate underwater in the horizontal lowered position.