Someone was targeting the security guard.

He felt he was being followed. His dog barked in the middle of the night. His wife saw fleeting figures lurking around their home. One night he awoke to find his front door open.

And now, to top it off, he was sick. Nausea rose up within him in waves as he made his rounds. He worked the night shift, which meant intermittent inspections from dusk to dawn, patrolling the grounds for any signs of trouble. The result was always the same: nothing.

He was the lone guard at the Advania data center, housed in a former U.S. naval base not far from the Reykjavík airport in Iceland. His job was to keep watch over two hangar-like buildings that held rows of small, box-like computers, the size of two cartons of cigarettes, stacked in towers as far as the eye could see. It was a hot, constantly blinking trove of devices, lashed together with tangles of cables and wires, all dedicated to a single job: mining the cryptocurrency known as Bitcoin.

Working around the clock, seven days a week, the computers were part of the largest concentration of Bitcoin mining power in the world. By solving and packaging complex “blocks” of encrypted data, the machines helped secure and expand the worldwide network of digital currency. And in return for their work, they generated vast fortunes for their owners. The Advania network alone, operated by Iceland’s largest IT provider, pulled in what’s estimated to be millions a year.

The night shift at the data center was the worst, the country plunged into darkness 19 hours a day by a stingy sun. Braced against the arctic cold on this January evening, the security guard was feeling sicker by the minute. Finally, around 10 p.m., he jumped into his car and sped home, rushing straight to the bathroom. “Diarrhea,” an attorney would later explain. When he emerged, he was too weak to walk. So he lay on the couch—just for a minute!—and immediately fell asleep.

Jolted awake just before seven the next morning, he rushed to his car to return to work, only to find that someone had slashed his tires. He called headquarters and was told to wait for backup. Just after noon, the guard, who had gone back to sleep, awoke to the sound of police officers pounding on his door.

While he was sleeping, someone had broken into the data center and stolen 550 Bitcoin computers, along with motherboards, graphics cards, and power accessories—a haul worth $500,000 for the hardware alone. It was the fifth cryptocurrency data center in Iceland to be hit in two months. The total take: $2 million in tech gear.