This video of an 8400-tonne steel structure — part of an oil platform — shows how it was launched in the Danish North Sea in under a minute, after a hell of a lot of preparation. It is 93m long by 60m wide, so about the size of a football pitch.
The trick was to fill the barge carrying it with water until it tipped just enough to help it slide off. Floating tanks then kept it buoyant until it was raised to a vertical position and finally lowered onto the seabed in just the right spot.
This video explains how the whole process works and the many stages still to be completed: