Officials at Poland’s Supreme Administrative Court in Warsaw discovered a number of high-powered cryptocurrency mining rigs hidden in the courthouse — including in a ventilation duct and beneath a raised floor — which had been powered by electricity from the court’s mains supply.

The devices had their own modems to connect to the internet, according to Polish news channel TVN 24, meaning they were not connected to the court building’s network.

According to TVN 24, the devices were capable of consuming several thousands Polish Zlotys of energy per month — with 1,000 Zloty worth about $250 — and had been placed near power supply equipment.