English police were surprised earlier this month when a raid on what they suspected was a cannabis farm uncovered a hidden cryptocurrency mining operation leeching electricity from the mains supply.

There were telltale signs that someone was growing copious amounts of weed in a unit at the Great Bridge Industrial Estate in Sandwell: people were coming and going at all hours, there was a strange amount of ventilation ducts and wiring, and when a police drone flew overhead it detected significant heat radiating out of the unit.

But officers barging into the premises on 18 May found the whirring of around 100 small computers very quickly solving cryptographic hashes instead of a room filled with hydroponic equipment and distinct smelling leafy plants.

“It’s certainly not what we were expecting,” said Sandwell Police sergeant Jennifer Griffin.

“It had all the hallmarks of a cannabis cultivation set-up and I believe it’s only the second such crypto mine we’ve encountered in the West Midlands.”

After making some inquiries with the Western Power electricity company – which has a distribution centre around the corner from the industrial estate – police learned the bitcoin miners were siphoning “thousands of pounds” worth of electricity to run the mine.

“My understanding is that mining for cryptocurrency is not itself illegal but clearly abstracting electricity from the mains supply to power it is,” Griffin said.

“We’ve seized the equipment and will be looking into permanently seizing it under the Proceeds of Crime Act.

“No-one was at the unit at the time of the warrant and no arrests have been made – but we’ll be making enquiries with the unit’s owner.”

A close-up of one the 100 connected devices used for bitcoin mining, supplied by the West Midlands Police, shows what appears to be a Bitmain Antminer S9, an older application-specific integrated circuit (ASIC) produced specifically for cryptocurrency mining.

These aren’t cheap pieces of equipment.

Depending on where you look online, used S9s go for anywhere between $350 and $1,000.

With an advertised hashing power of 13.5 terahashes per second (TH/s), an S9 mining at the current hash rate would make about 0.002 bitcoin per month.

At the current bitcoin price of $46,000, 100 of these S9s powered by stolen electricity could make its owners around $9,200 worth of bitcoin each month.