The structural remains of the timber section of the First World War boom defence between Grain and Sheerness to protect the Medway from naval attack. The boom was stretched between Grain Tower and Garrison Point on Sheerness. Between Grain Tower and the mainland of the Isle of Grain a wooden pier-like structure was constructed to protect the area exposed at low tide. This part of the boom could still be seen on RAF aerial photographs taken in 1942, but has since been removed. However, the eroded stumps of its posts can still be seen in the mud at low tide. The chain for the main boom still remains wrapped around Grain Tower. The wooden section of the boom was mapped from aerial photographs as part of the English Heritage: Hoo Peninsula Landscape Project.