Planet Earth is a 2006 British television series produced by the BBC Natural History Unit. Five years in the making, it was the most expensive nature documentary series ever commissioned by the BBC and also the first to be filmed in high definition. Planet Earth premiered on 5 March 2006 in the United Kingdom on BBC One. It had been shown in 130 countries worldwide by June 2007. Its first broadcast in the United States was on 25 March 2007 on the Discovery Channel, with Sigourney Weaver replacing David Attenborough’s original commentary. The series comprises eleven episodes, each of which features a global overview of a different biome or habitat on Earth. At the end of each fifty-minute episode, a ten-minute featurette takes a behind-the-scenes look at the challenges of filming the series.