A Pennsylvania-based holding company called Stronghold Digital Mining bought an entire coal power plant in Venango County, Pennsylvania over the summer to power 1,800 mining computers, NBC News reports.

The plant is now burning about 600,000 tons of coal a year, a troubling return to a highly polluting form of energy and raising red flags among environmental advocates.

“These miners don’t just need cheap energy, but a stable source of power because their machines need to run 24/7, and fossil fuel sources are best suited for it,” Alex de Vries, a Dutch economist, researcher and founder of Digiconomist, told NBC.

Stronghold is now planning to expand its operations by buying two more coal waste plants in the area, according to NBC. Coal waste plants use up environmentally hazardous piles of coal waste, byproducts of mining operations that can leach contaminants into waterways — but burning it still emits plenty of greenhouse gases.

While cryptocurrency mining operations are flourishing stateside, other parts of the world are actively trying to stop it, like China, which declared all crypto transactions illegal on Friday.

A major concern driving that crackdown in China is the astronomical amount of energy being used to mine crypto. The country aims to become carbon neutral by 2060.

Companies like Stronghold are returning to coal, a source of electricity that was recently surpassed by renewables for the first time in US history last year, because it’s cheap — environment be damned.

“Miners are reviving gas plants and idle coal mines in places like New York and Montana,” de Vries told NBC.

Stronghold is now preparing for its initial public offering. The company maintains it is “environmentally beneficial” in Securities and Exchange Commission filings, pointing to the fact it is using up coal waste.

“Simply put, we employ 21st century crypto mining techniques to remediate the impacts of 19th and 20th century coal mining in some of the most environmentally neglected regions of the United States,” the company wrote in its SEC filing.

But environmentalists argue there are better alternatives to dealing with waste coal, such as diverting it to lined landfills or planting sea grass on top of the piles.

Local legislatures, however, appear to be more in favor of cutting taxes for cryptocurrency mining operations than investing in cleaning up the environment.