In 1953 a very bad flood covered 160,000 acres on Canvey Island and killed 300 people in Essex. That forced the UK government to appoint a committee to look at flooding.
The solution was the Thames Barrier, built at a place called Woolwich Reach.
It is nine concrete barriers and between each is a large metal gate that can be raised when needed to guard against flooding.
Apart from monthly tests, the Barrier has been raised 25 times since it was finished in 1982.