Explosions at a U.S. Steel plant that shook the ground near Pittsburgh left one dead and sent at least 10 to hospitals with emergency workers still searching the badly charred rubble hours later for a missing worker.

An Allegheny County spokesperson says the explosions happened Monday morning at the Clairton Coke Works. The plant is a massive industrial facility along the Monongahela River south of Pittsburgh, and is considered the largest coking operation in North America.

The explosions sent black smoke spiralling into the midday sky in the Mon Valley, a region of the state synonymous with steel. Officials said a rescue operation was ongoing Monday.