With oil prices over $100 a barrel this week, the companies involved in Canada's tar sands must be rubbing their hands in glee. The $26 a barrel cost of processing (compared to about $1 in Saudi Arabia) suddenly doesn't look so bad. Oil companies who have chosen not to invest may be tempted to reconsider, and that's very bad news.

The Athabasca tar sands, in Alberta, may be the world's largest oil reserve. Only the surface sands are accessible at the moment, but if the technology develops a little more, there's potentially six times more oil there than the whole of Saudi Arabia - enough to last 200 years, say the champions of the project.

But, it's not liquid oil, and extracting the crude from the sand takes vast reserves of water, a quarter of Alberta's fresh water. This water is so polluted at the end of the process that it is simply left to stand in huge tailing pools that altogether cover some 50 square kilometres. It's so toxic that birds landing on the ponds would die. Some places use propane cannons to scare the ducks away; others just rake the dead birds off the surface. As the ponds aren't lined, waste water leaks into the Athabasca River, polluting everything downstream - lakes, deltas, and the Mackenzie River.

It also destroys the land. Huge areas of the boreal forest ecosystem have been felled and the underlying peat bogs cleared away to expose the sands. At the end of the processing there is nothing but a 'toxic moonscape' of earthworks, ponds, and 80 foot high piles of pure sulphur. 5,000 hectares have been destroyed already, and David Schindler of the University of Alberta estimates that in ten years time they will have cleared an area the size of Florida.

The air is not spared either. It takes enormous amounts of heat to extract the oil, approximately a barrel of gas for every two of crude. The total emissions of the tar sands project will soon be equivalent to the whole of Denmark. Acid rain falls all across Alberta and now Saskatchewan too. In the summer, the tailing ponds release carcinogenic benzene. "If the wind is from the north-west," writes Aida Edemariam of the nearby boomtown of Fort McMurray, "you can smell oil on the air: heavy, slightly sour, unmistakable."

All of this makes the tar sands 'the most destructive project on earth' (pdf), according to last week's report by Environmental Defence.

Having read last week that the site was visible from space, I tracked it down on Google. It's right here if you want to explore it and see for yourself.