4 people died when the dock gate (caisson) keeping out the River Medway failed and flooded the dock where HMS Talent was undergoing repairs. The dock flooded pushing the submarine off its supporting wooden blocks and allowing it to float free out into the River Medway where it beached on some mud flats . There is a British Pathe newsreel of the event here.
A caisson works by emptying and filling it with water, when you wish to flood the dock you empty the tanks in the caisson allowing it to rise out of its place, if you wish to seal the dock you pump water into the caisson tanks and allow it to plug the entrance of the dock. On this occasion workers needed access into the caisson for repairs, to do this the flooding holes were plugged which changed the pressure in the caisson but allowed the workers access, the bottom of the caisson was flooded to try and counteract the change in pressure as had been done on a previous occasion. Unfortunately where as it worked the previous time it had not been taken into account that at this time there was an unusual high tide causing the caisson to become buoyant and as a result water rushed into the dock.
This dock is now occupied in the museum by HMS Ocelot.
there are more articles below:-