The moat surrounding the Tower of London was originally built by Henry III as a defensive ditch rather than a moat. It was not until the 13th century that it was connected to the Thames and flooded with water. However over time, the changing level of the river meant that there was little water flow between the river and the moat meaning that the moat water became stagnant. In the 19th century, the Duke of Wellington, concerned at the health risk the stagnant moat posed to the garrison ordered it drained and it has remained dry every since.
It has been used as a site for filming and in recent winters as the site of an ice skating rink, but perhaps most memorably as the site of the ceramic poppies display during the World War 1 commemorations in 2014.