A group of thieves in the UK dug a 50-foot underground tunnel to steal money from an ATM this week, according to a statement released today by the Greater Manchester Police. The theft occurred between midnight and 6AM on March 14th, at a Tesco grocery store in the town of Eccles. Police say the thieves accessed and escaped the store from an underground tunnel that they dug at an abandoned lot nearby. It's not clear how much money they made off with, though investigators described it as "a substantial sum." Given the complexity of the heist, police suspect that it took months for the perpetrators to plan and execute.

"The offenders must have spent long periods of time in the area over the last few months, which people may have noticed," Mark Toker, a detective with the Greater Manchester Police, said in a statement. "You may have seen people acting suspiciously on Friday night, possible [sic] covered in soil. I would ask anyone with any information about the robbery to call us as soon as possible."

This isn't the first time that thieves have tunneled their way into British cash machines. In 2012, a group dug a 100-foot tunnel to steal about £6,000 ($9,915) from an ATM located near a Blockbuster store in Manchester, cutting their way through 15 inches of concrete and meticulously covering up their tracks. The same group, known as the "mole gang," is believed to have attempted a similar heist near the same shopping center in 2007, before the plot was foiled by police. It's not yet clear whether the mole gang was involved in last week's theft.