The Thames Barrier is a massive civil engineering solution to the danger of flooding in the London boroughs caused by high tides and storm surges in the North Sea. A series of sci-fi, stainless steel control units march boldly across the river.
You can find the barrier on the River Thames close to London City Airport.
The barrier is closed when water is surging up the Thames towards London and also when flood waters are expected from further up the Thames. In the latter case, the gates are closed at low tide to form a large empty space upriver of the gates to hold the flood waters arriving from upstream.
The Visitor Centre is located at the southern end of the barrier. On the northern side of the river, a good view of the barrier is available from the Thames Barrier park close to Pontoon Dock DLR station.
Some extended Greenwich river boat tours make the trip down to the barrier.