In a town called Lago Agrio, “sour lake,” deep in the oil-rich rain forest of Ecuador, what could be the biggest environmental lawsuit in history is being fought.

It’s a battle pitting 30,000 indigenous Ecuadorians, and a team of American trial lawyers, against the U.S. oil giant Chevron. The plaintiffs claim Chevron is responsible for the damage caused by 18 billion gallons of toxic oil waste, and an Ecuadorian court-appointed expert recommends the company pay up to $16 billion.

Chevron says the case is fundamentally flawed. Activists call it a global game-changer.