Coal production in Colorado fell by nearly 40 percent in 2016, dropping to a low the state hasn’t experienced since the 1970s.

The Daily Sentinel reports that the U.S. Energy Information Administration released data Thursday showing the state’s 2016 production was 11.4 million tons, down from 18.9 million tons in 2015, or a 39.5 percent decline.

Across the country, production was similarly down, falling 17.6 percent to 739 million tons.

The federal data is an estimate. Year-end production numbers calculated by the Colorado Division of Reclamation, Mining and Safety are not yet available.

The dip in production comes in part due to tumultuous conditions at Colorado’s coal mines. The owners of the state’s two highest-producing mines both dealt with bankruptcies in 2016 and Peabody Energy’s Bowie No. 2 Mine was idled early in the year due to poor market conditions.