Hollywood star Mark Ruffalo on his new film ‘Dark Waters’

They are nicknamed “forever chemicals.” Industrial chemicals that most people have never heard of – yet they’re found in vast amounts of drinking water across the United States.

A new film called Dark Waters tells the shocking real life story of a lawyer who took on the case of one farmer, convinced his cows were being poisoned by a nearby DuPont chemical factory.

Jon Snow has been to meet lawyer Rob Bilott – and Mark Ruffalo, who plays him in the film.