Protests opposing the construction of an oil pipeline through four northern US states have been drawing activists at sites around the country for weeks, including at the White House, in Atlanta, Cleveland, and Los Angeles.

At the White House, former presidential candidate Sen. Bernie Sanders (D-VT.) attended, saying the pipeline “must be stopped.”

The protesters included many members of the Standing Rock Sioux Tribe, which filed a lawsuit on July 27 to halt the pipeline’s construction and have been rallying opposition against it.

Here’s what you need to know to understand what’s at stake during these protests.

What Is the Dakota Access Pipeline?

A Texas-based energy company called Energy Transfer Partners has proposed a 1,100-mile pipeline to move crude oil through the Bakken formation, the name for a large swath of rock and shale containing oil and gas deposits in North Dakota, Montana, and Canada.

The Dakota Access Pipeline would take oil from North Dakota through South Dakota and Iowa to Illinois, where it could then be taken by train or truck to markets throughout the country or transferred to a second, existing ETP pipeline and taken to the US gulf.

The pipe, which will be built mostly on privately owned lands, will transport nearly half a million barrels of crude oil per day once it is opened.

The company has said no damage will be caused to the surrounding area or water sources, and has received approval from the U.S. Army Corps of Engineers to signify that it will have “no significant impact” on historic landmarks.

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Who Opposes It and Why?

The Standing Rock Sioux Tribe’s land in North Dakota is about a half-mile away from the pipeline’s proposed route. The tribe also relies for its water on a lake called Lake Oahe, a reservoir off of the Missouri River along pipeline route. Earlier this summer, the tribe sued to stop the pipeline’s construction.

“The pipeline threatens our sacred lands and the health of 17 million people who rely upon the Missouri River for water,” the tribe said.

The lawsuit also claimed the construction would destroy burial grounds that carry “enormous cultural importance to the tribe.”

The tribe has received support from other Native Americans across the US as well as support from the public, including from celebrities like Shailene Woodley and Susan Sarandon, according to Reuters. Sanders joined the fight against the pipeline last week.

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"We cannot allow our drinking water to be poisoned so that a handful of fossil fuel companies can make even more in profits," Sanders said at the protest in Washington.

Why Are They Protesting This Week?

Last week, a federal judge overseeing the Sioux Tribe’s case denied a motion to temporarily halt construction.

The Obama administration, including the Departments of Justice, the Interior and the Army, then announced it would halt the pipeline’s construction on federal land and revisit its approval of the project.

In response, ETP sent out a letter saying it was committed to building the pipeline, had already spent $1.6 billion and was 60 percent done with construction, had acquired all of the permissions to build the pipeline, and was building it along an existing natural gas pipeline.

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“Concerns about the pipeline's impact on the local water supply are unfounded. Multiple pipelines, railways, and highways cross the Missouri River today, carrying hundreds of thousands of barrels of oil,” the company said.

On Sept. 13, 22 people were arrested while protesting at the pipeline site in North Dakota, two of whom attached themselves to construction equipment, police told the Bismark Tribune.

What Happens Next?

The Justice Department said it would review its approval of the pipeline to see whether it followed all federal environmental laws in allowing it to move forward.

“The Army will not authorize constructing the Dakota Access pipeline on Corps land bordering or under Lake Oahe until it can determine whether it will need to reconsider any of its previous decisions regarding the Lake Oahe site under the National Environmental Policy Act (NEPA) or other federal laws,” it said.

The DOJ promised to move “expeditiously,” noting that that the pipeline company and its workers deserved a “clear and timely resolution.”