In a student’s bedroom you would expect to find unread books, unwashed plates, and an unmade bed. A cryptocurrency mine probably doesn’t make the list. But last month, security researchers at tech conglomerate Cisco found that 22 percent of cryptocurrency mining traffic comes from college campuses.

Mining malware could play a part—but a major factor is thought to be the peculiar economics of mining for students. Many students living in campus accommodation have their electricity bills covered by their university. They seem to be taking advantage of the free electricity to leave cryptocurrency mining rigs running, at no personal cost apart from the hardware itself.

As the crypto winter drags on, the cost of electricity outweighs the value of the mined cryptocurrency for many. Not having to pay their own bills, students have a rare advantage.

“If you don’t have to pay for those costs, then you are in a really good spot for making money on the university’s dime,” Cisco researcher Austin McBride told PC Mag.

When cryptocurrencies were booming in value, students found themselves in an even more enviable position. When bitcoin was valued at more than $10,000 in January last year, Stanford University posted on its website about a “sharp increase in incidents involving cryptocurrency mining.”

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Students are generally prohibited from mining, as they are using university resources for personal financial gain. The Stanford post cites six university policies that the students may be breaking. But with limited IT budgets, institutions have not been able to enforce those rules.

College education came in second as a source of mining traffic after the energy and utilities industry. McBride told PC Mag this could be because systems in those industries are often purpose-built, which means they are updated less frequently. That makes them more vulnerable to infection by malware using their power to mine.