MTV has posted a new interview with production designer Stuart Craig where he talks about filming Harry Potter and the Deathly Hallows.  He discussed the challenges of filming the Sword of Gryffindor scene.

Another one of the production’s major challenges — and accomplishments — was shooting the sequence in which Harry retrieves the Sword of Gryffindor at the bottom of a frozen lake. “There was a really demanding, complicated special-effects requirement there to do the ice,” Craig said. “I think that all works remarkably quite well, actually. Harry breaking the ice, diving in and then subsequently strangled by the Horcrux around his neck and is struggling and can’t get up quickly because of the ice above him. It’s good stuff.”Which begs the question: How did Craig and his team pull off that scene, and what do they use to make the ice look so real? “As always, well, as nearly always, there’s more than one solution. The camera on top, looking from the outside down on it. It’s big, thick sheets of Plexiglass with frosty texture on top of that,” he revealed. “When we’re underneath, it’s actually an area of wax which floats on top of the water. And wax makes very effective ice. They’re tried and tested movie techniques; there are a lot. You could write a book one day, a guidebook, to the very movie techniques — frost on window panes with some Epsom salts and brown nails.”