Billionaire investor George Soros has opened new equity stakes in Peabody Energy Corp. and Arch Coal Inc., the country's top two coal producers, according to a Form 13F-HR filed Aug. 14.

Soros acquired more than 1 million shares of Peabody and 553,200 shares of Arch in the second quarter, according to the filing. He reported no other coal holdings in the period.

Soros makes investments through his Soros Fund Management LLC fund. Soros' holdings may have changed significantly since the end of the second quarter ended June 30.

Soros last invested in the coal sector in 2014 when he opened a large stake in coal and gas producer CONSOL Energy Inc., but he later liquidated it.

Both Peabody and Arch have seen their market values plummet amid extremely difficult operating conditions for domestic producers caused by competition from cheap natural gas, new environmental regulations and a slowing export market.