A man has been arrested and is facing charges in a small town outside of Boston after local authorities discovered “an illegal cryptocurrency mining operation” tucked away under a school’s floor.

As the BBC report, the accused—39-year-old Nadeam Nahas—was previously the assistant facilities director for the small town of Cohasset, which is roughly 10 miles outside Boston. In December 2021, a “routine inspection” of the school turned up some suspicious modifications.

Cohasset Police Chief William Quigley told the BBC that, according to Cohasset High School’s own director of facilities, “during a routine inspection of the school he noticed electrical wires, temporary ductwork, and numerous computers that seemed out of place”.

The ducts and wiring were all headed towards the same place, “near the school’s boiler room”, and checking under the floor there “a cryptocurrency mining operation that was illegally plugged into the school’s electrical system” was discovered.

As you can see in the image above, provided by local police, while this wasn’t a warehouse-sized operation—11 computers in total were discovered, including some “inside ‘coolers’ that vented to the outdoors”—that was still a significant amount of hardware to be found hiding under a school’s floors. And it was drawing more than enough power to leave a trace.

Authorities say that the mining setup operated from April to December 2021, and racked up electricity bills for the school estimated at around $17,500. Nahas, who plead not guilty in a court appearance last week, resigned from his job as assistant facilities director shortly after the operation was discovered. He was arrested last year after a months-long investigation that involved not just local police, but the US Coast Guard and the Department of Homeland Security as well.

The Boston Globe say Nahas’ specific charges are vandalizing a school and fraudulent use of electricity, and he has been “ordered to stay away from all public buildings in Cohassett”.