Like invading giant, metallic, alien tripods, these relics from World War Two loom out of the water at the mouth of the Thames Estuary near London. They were Army Maunsell Sea Forts, and their purpose was to spot and fire at German bombers.
These forts had a garrison of over 400 men and were nearly 80 feet tall. They were decommissioned in the 1950s. Several still remain and they are definitely a weird and creepy sight. There were also some built on the River Mersey outside the city of Liverpool, but all were demolished after the war.