A shipyard in South Korea is building the biggest vessel the world has ever seen - a gas processing platform for the energy giant Shell.

Longer than four football pitches laid end-to-end, it stretches for nearly half a kilometre (1600 ft).

Known as Prelude, the vessel will eventually be moored above a gas field in remote waters off Australia and is designed to provide a new way to get gas to the global market.

BBC News was given exclusive access to the construction site and Science Editor David Shukman had the chance to look around.