Without it, London would likely be a very wet place.
Since its official opening in 1984, the Thames Barrier - one of the world's largest movable flood barriers - has protected the capital from the threat of tidal surges and catastrophic flooding.
The city could have done with it back in 1928 when a flood caused the loss of 14 lives and left 4,000 homeless. As the years passed, it became apparent that such a disaster could happen again, and possibly on a far greater scale. And so the Thames Barrier was built.
Spanning 520 metres, the barrier comprises 10 steel gates, each one weighing a mighty 3,300 tonnes.
The barrier has already been utilised more than 100 times and as recently as March it was raised three times in two days. The city is most at risk when certain conditions combine – such as a storm surge and a high spring tide.
With rising sea levels, the threat of flooding is bigger than ever. Indeed, because of the rising sea levels, this Thames Barrier will not be a permanent fixture. It's predicted that by 2070, a new structure will be needed.
The barrier has a fascinating visitor centre, located on the Thames' southern side, which provides information about how the barrier was designed, built and operates, and also details the history of England's most famous river.
As well as the visitor centre there's also a cafe, an outside picnic and play area, and impressive views of the barrier itself.
The nearest tube station is North Greenwich on the Jubilee Line (about two miles from the barrier), or for rail travellers there's Charlton train station, about a mile away.
The Thames Barrier is an awesome feat of contemporary engineering and undoubtedly the city's unsung hero, over the years preventing the loss of thousands of lives.