SM UB-122 was a World War One German submarine, or U-Boat, in the German Imperial Navy which was first commissioned in March 1918 after a year of construction. The 183ft warship was armed with 10 torpedoes and a 3.46 inch deck gun which would have been operated by the 31 men and 3 officers onboard.
Within just nine months UB-122 was one of 100 warships to be surrendered at the end of the war. It was common for the ships to be stripped of anything valuable and the engines from this U-Boat were taken to Halling and were used at the cement works there; possibly even the site that we visited.
Marine archaeologist and maritime designation adviser for English Heritage, Mark Dunkley, has said that they’re unsure on how the ship found its way to the Medway however it’s likely that it became adrift whilst being towed and ended up getting stuck in the mud off of the Hoo Peninsular and would have been too much effort to move it. Speaking in 2013 he said that English Heritage have no plans to conserve it.
Sources: Heritage Daily, Daily Mail