They flew over the mined area, a region about the size of Florida that looks like the aftermath of a meteor impact. There are known reserves of 174 billion barrels of oil in these sands – a volume exceeded only by Saudi reserves.

On their journey, the group met local and indigenous people living downstream from the oil sands development and heard their stories of frustration and fear. Waters are now polluted, traditional livelihoods lost and rates of unusual cancers have increased.

The oil beneath Alberta's ancient forests was once considered too expensive and difficult to extract. But in their desperation for fuel, oil giants have resorted to these 'unconventional' oil supplies, the production of which generates three times the level of greenhouse gas as conventional oil. WWF is calling for a freeze on further oil sands developments.

• Watch the ITN report on oil sands in Canada.
• Read about WWF's work with the oil and gas sector.
• Carbon-crazed Canada on course to fail Kyoto.
• Canada joins US as worst performer on climate change.