Production at one of the largest mines in the U.S. hasdropped to the lowest quarterly level of the millennium.

Data provided by the U.S. Mine Safety and HealthAdministration indicates that ArchCoal Inc.'s Black Thunder mine in Wyoming produced only 13.7million tons in the second quarter 2016, the lowest amount since the lastquarter of 1999 when the mine produced around 13.2 million tons.

The recent quarter's number is down 9.6% from the 15.2million tons reported in the first quarter of 2016 and decreased 41.5% from the23.5 million tons produced in second quarter 2015.

Production has dropped more than half from the 31.4 milliontons produced in the third quarter of 2010 — the highest production quarterreported in this millennium.

Arch did not respond to a request for comment in time forpublication, but the company is reorganizing under Chapter 11 and has shared in thechallenges many coal companies in the U.S. are facing.

Data also indicates the average number of employees at themine is down to 1,278 employees working in the mine in the most recent quarter— the lowest number of personnel since the last quarter of 2009. Arch hadannounced it wascutting 15% of Black Thunder staff in March due to "persistent weakness"in the market for thermal coal.

The number of employees on average reported in the firstquarter 2016 was 1,541, but the outlook for staff at the mine is grim if thistrend continues. The last time quarterly production was so low, Black Thunderemployed an average of 513 miners. That was in the fourth quarter 1999.