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As green groups and Indigenous leaders continue to raise alarm about the ecological and economic threats of the Trans Mountain pipeline expansion project—which Canadian Prime Minister Justin Trudeau recently announced the government is taking over after protests led Kinder Morgan to halt construction—12 activists on Tuesday launched an aerial blockade at the Ironworkers Memorial Bridge in Vancouver to stop an oil tanker from leaving the pipeline’s terminal.

Opponents of the expansion project are especially concerned that, if completed, it would trigger a nearly seven-fold increase in the number of tar sands tankers that depart from the company’s terminal in Burnaby, British Columbia, increasing the risk of a major oil spill and degrading marine conditions along the “tanker superhighway.”

Greenpeace Canada’s Mary Lovell, a climber from Seattle, Washington, said in a statement: “We urgently need to protect water because this tanker superhighway threatens the entire coast down to California. We are taking action today to defend water, health, and the climate—and we aren’t alone. Hundreds of thousands of people from around the world are standing with us.”