Keira Knightley, the water nymph

The secret behind Keira Knightley''s underwater shoot is finally out.

The actress was snapped in a special 1.2 million litres tank built tank at Pinewood Studios that has also been used to film sequences in action flicks like The Bourne Ultimatum and Casino Royale.

The Telegraph quoted photographer Phoebe Rudomino, as saying about Knightley: 'She was naturally very good. She was wearing a fairly heavy dress which made it hard but she was surrounded by safety divers and she did very well.'

The 24-year-old beauty was shooting to promote the charity Fresh2o, which provides clean water to people.

Knightley said: 'There may be gaffer tape holding a bikini together or divers hanging upside down to keep someone in a certain position but the viewer will only ever see the final polished shot.'

The photos will now be showcased in a forthcoming exhibition called Water on the Lens, in London in October.