Thieves dug a 50ft tunnel under a building to steal thousands of pounds from a cash machine in Salford, Manchester.
Police believe the gang may have spent months digging the "complex" structure to get at the cash at a Tesco store in Eccles.
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 a number of 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.
"You may have seen people acting suspiciously on Friday night, possibly covered in soil.
"I would ask anyone with any information about the robbery to call us as soon as possible."
Anyone with information is asked to call police on 0161 856 5353 or call Crimestoppers anonymously on 0800 555 111.
It is the second time a tunnel gang has struck in Greater Manchester.
A Blockbuster shop in Levenshulme, Manchester, eight miles away from the Salford store, was raided in January 2012, after a gang spent months building a 100 foot tunnel with lighting and roof supports.
Police said they may have spent six months digging the passageway from a railway embankment, under a car park and beneath the shop where the cash machine was.