 Complete the following phrase:  The main attraction for tourists is a prototype of the World War II Barnes Wallis Highball bouncing bomb that was tested in the sea off Reculver between 6 April and 13 May 1943. The location was chosen because the shallow water allowed easy recovery at low tide, and it was secure and close to RAF Manston. To imitate the weight of explosive, the designers filled the Highball with checol: concrete and chalk. The Army found this exhibit on the seabed in 1997, and it has been conserved for display. The Upkeep bouncing bomb, a later development of this prototype, was used in the 1943 