HMS Artful: Time-lapse video shows submarine entering water
21 May 2014 Last updated at 11:55 BST
Time-lapse footage has been released of HMS Artful being lowered into the water in Cumbria.
The third Astute Class submarine is one of seven nuclear-powered vessels being built by BAE Systems in Barrow.
The 97m-long 7,400-tonne attack submarine, which was officially named at a ceremony in September, will now undergo tests to prove its systems work properly before sea trials in 2015.
BAE Systems said it was a significant step forward in the Astute programme.