A new Thames Estuary Airport has been proposed at various times since the 1940s. London's existing principal airports, Heathrow, Gatwick and Stansted, are each sub-optimally located in various ways, such as being too close to built-up areas or requiring aircraft to fly low over London. In the case of Heathrow, the growth of air traffic has meant that the airport is operating at 98% capacity. Several locations for a new airport have been proposed in the Thames Estuary, to the east of London. These include Maplin Sands off Foulness on the north side of the estuary; Cliffe and the Isle of Grain in Kent on the south side; and artificial islands located off the Isle of Sheppey such as the "Boris Island" proposal championed by Boris Johnson, the then Mayor of London. Economic considerations have