Buried under Canada’s boreal forest is one of the world’s largest reserves of oil. Bitumen—a very thick and heavy form of oil (also called asphalt)—coats grains of sand and other minerals in a deposit that covers about 142,200 square kilometers (54,900 square miles) of northwest Alberta. According to a 2003 estimate, Alberta has the capacity to produce 174.5 billion barrels of oil.

Only 20 percent of the oil sands lie near the surface where they can easily be mined, and these deposits flank the Athabasca River. The rest of the oil sands are buried more than 75 meters below ground and are extracted by injecting hot water into a well that liquefies the oil for pumping. In 2010, surface mines produced 356.99 million barrels of crude oil, while in situ production (the hot water wells) yielded 189.41 million barrels of oil.

Since then, oil sands production has outpaced conventional oil production. While conventional oil has held steady, oil sands output has gone up from 1 million barrels per day in 2005 to 2.4 million barrels per day in 2015.

These images from Landsat satellites show the growth of surface mines over the Athabasca oil sands between 1984 and 2016. The Athabasca River runs through the center of the scene, separating two major operations. To extract the oil at these locations, oil producers remove the sand in big, open-pit mines, which are tan and irregularly shaped. The sand is rinsed with hot water to separate the oil, and then the sand and wastewater are stored in “tailings ponds,” which have smooth tan or green surfaces in satellite images.

The process of extracting oil from the sand is expensive. It takes two tons of sand to produce one barrel of crude oil. Great Canadian Oil Sands opened the first large-scale mine in 1967, but growth was slow until 2000 because the global cost of a barrel of oil was too low to make oil sands profitable.

The images above show slow growth between 1984 and 2000, followed by a decade of more rapid development. The first mine (from 1967, now part of the Millennium Mine) is visible near the Athabasca River in the 1984 image. The only new development visible between 1984 and 2000 is the Mildred Lake Mine (west of the river), which began production in 1978, and was expanded in 1996.

After 2000, the price of oil began to climb, and investment in oil sands became profitable. The Millennium Mine expanded east of the Athabasca River, and the Steepbank Mine was developed in the east. The Mildred Lake Mine also shows evidence of growth. It is a trend that is likely to continue since permits have been approved to expand mines in this region. The large images include a view of additional mines developing to the north.

Oil sand mining has a large impact on the environment. Forests must be cleared for both open-pit and in situ mining. Pit mines can grow to more than 80 meters depth, as massive trucks remove up to 720,000 tons of sand every day. As of September 2013, roughly 895 square kilometers (345 square miles) of land had been disturbed for oil sand mining.

Companies are required to restore the land after they have finished mining. In this series of images, the large tailings pond from the 1967 mine was gradually drained and filled in. The mining companies pledged to plant grasses on the site. Satellite imagery shows greening after 2011.

Tailings ponds contain a number of toxins that can leak into the groundwater or the Athabasca River. The mining and extraction process releases sulfur oxides, nitrogen oxides, hydrocarbons, and fine particulate matter into the atmosphere.

Both water and air pollution could pose a health hazard, but an independent panel of experts found no definite connection between the mines and specific illnesses as of December 2010. The panel warned, however, that the mines may impact health as they expand over coming decades.

Because it takes energy to mine and separate oil from the sands, oil sands extraction releases more greenhouse gases than other forms of oil production. The mines shown in the image emitted more than 20 million tons of greenhouse gases in 2008— a product of both oil production and electricity production for the mining operation. The effort produced the equivalent of 86 to 103 kilograms of carbon dioxide for every barrel of crude oil produced. By comparison, 27 to 58 kilograms of carbon dioxide are emitted in the conventional production of a barrel of crude oil.

For all of these reasons, some groups have labeled the oil sands an environmental menace. On the other hand, they offer a stable source of energy and economic growth. The Athabasca oil sands are the largest segment of the economy in Alberta, making up just over 30 percent of the gross domestic product. In the 2013–2014 fiscal year, the Alberta government reported that “synthetic crude oil and bitumen royalty [from oil companies] accounted for about $5.2 billion or almost 55 per cent of Alberta’s $9.6 billion non-renewable resource revenue.”

The economic benefits extend beyond Alberta. Canada is a leading source of oil for the United States, the world’s largest consumer of oil. The oil sands contain enough oil to produce 2.5 million barrels of oil per day for 186 years. In 2015, the U.S. consumed 19.4 million barrels of oil per day.

TransCanada has proposed building a pipeline to bring oil from the Athabasca oil sands directly to refineries in the United States. But the pipeline, known as Keystone XL, has not been approved by the U.S. government.