A 4-by-8-mile pit deep in the wilderness of northwest Canada is taking center stage in America’s energy debate. The Athabasca tar sands in Alberta Province is the largest mine in what is the second largest oil reserve in the world, behind only Saudi Arabia.

The United States gets 20 percent of its imported oil from Canada, about half of it coming from the vast Athabasca tar sands. And now there are plans to more than double the oil sands production and pipe nearly all of the oil through a new pipeline that would take it to refineries in Texas.

Recovering the estimated 175 billion barrels of oil is costly and controversial. The Athabasca tar sands produce a low-grade asphalt-like petroleum product, called bitumen, that must be melted out of the ground, a process that environmental groups say produces three times more carbon dioxide than traditional oil drilling. But the pipeline needs U.S. approval all the way up to the State Department, a decision that is expected to made later this year.

The Enemies Of America: The Sierra Club has dubbed tar sands as “the most toxic form of oil on earth,” and the organization will join a coalition of environmental groups planning to protest the importation of this oil at the White House starting Aug. 20.

Among the concerns by green groups are leaks and the environmental impact from the proposed pipeline, which would cross several major watersheds and aquifers.