It looks like a stunning piece of architectural art, but its purpose is deathly serious. For centuries London lived in a delicate balance with the River Thames. The river brought food, transportation, communication, and commerce to the city. But occasionally the river would exact a toll from the city, overflowing its banks and taking lives and property in return. One of those floods came in 1953. Three hundred people lost their lives downstream from the city, and thousands of acres of productive farmland were rendered useless by the salt water. London's response was seemingly simple -- a wall separating London from the ravenous sea. But the simple idea was complicated to implement. The Tidal Barrier had to open and close quickly. It had to permit significant water flow in both directions to keep the ecological balance. And it had to allow ships to pass without encumbrance. The result is a line of ten gates that can be raised and lowered at will. Underneath each of the barrier's stainless steel domes is a machine much like a construction crane. When it moves its arms, a massive horizontal drum rotates. This drum had a large notch in the center of it. When the arms are extended, the drum rotates so the notch faces up, allowing ships and water to pass through it. When the arm retracts, the drum turns on its axis and the un-notched side of the drum rises out of the water, blocking the river. Of course, all that water has to go somewhere, so the banks of the Thames have been raised for 50 miles to protect people downstream from being inundated. More importantly, even when a serious threat is predicted, the gates are closed slowly. Closing the barrier at its maximum rate would create a wave in the river that could do more damage. There are four main gates at the center of the river. Each is 200-feet / 61 meters long. Those are flanked by a pair of 102-foot / 31 meter gates. And on each end are two more gates which allow water, but not ships to pass.

Photograph © London Tourist Board