Thieves dug a 50ft (15m) tunnel under a building to steal thousands of pounds from a cash machine.
Police believe the gang may have spent months digging the "complex" structure to get at the cash at a Tesco store in Eccles, Greater Manchester.
They tunnelled beneath nearby wasteland and under the shop before raiding the store in Liverpool Road between midnight and 6am last Friday, March 14.
They then stole cash boxes containing a "substantial" sum of money before disappearing back through the tunnel with the cash, Greater Manchester Police (GMP) said.
Officers said they believe the thieves may have spent months digging the tunnel because of the amount of planning that was involved and complex nature of the tunnel structure.
Detective Superintendent Mark Toker from GMP said: "These people had obviously spent a long time plotting this crime and I doubt they would have been able to keep their plans secret for all that time, without telling others about their elaborate scheme.
"The offenders must have spent long periods of time in the area over the last few months, which people may have noticed."