People forget that London floods. Parliament has been under water, and in 1953, surges killed 307 people in the U.K. At least, London used to flood. The Thames Barrier is the city’s primary defense against it, comprised of ten 20m (66-ft.) steel-and-concrete gates that can be raised to block the 520m (1706-ft.) span of the river in just 10 minutes. Most of the time you can’t see the gates, which rest on the riverbed, but the piers that raise and lower them are always visible, strung across the river like a row of mini-Sydney Opera Houses. At the visitor center on shore, you plumb the Barrier’s construction and, if you’re lucky, see a test raise.