Plaintiffs seeking reparations in the U.K. for a deadly disaster in Brazil started their case in a London High Court Monday by asserting that mining giant BHP allowed an excessive amount of toxic tailings waste to be dumped at a dam that later collapsed.

The long-awaited trial stems from a 2015 accident at the Samarco iron ore complex, which is jointly owned by BHP. Plaintiff's firm Pogust Goodhead represents around 620,000 claimants who are seeking £36 billion (US$46.7 billion) in damages.