Authorities in South Korea’s capital Seoul have broken up what they’re calling the largest illegal gambling operation uncovered to date.

On Wednesday, the Seoul Metropolitan Police Agency announced they’d raided seven apartments in the southern part of the city that housed illegal gambling operations. In the process, police arrested seven gang members while 69 other staffers and 11 gamblers were booked without being detained.

Police said the operation had begun in 2011, with the illegal gambling parlors disguised as business offices to throw off suspicion. The cautious organizers reportedly shifted their locations every two or three months in a further bid to avoid detection.

One staffer told police that the ring controlled 70% of all illegal gambling dens in Seoul. At its peak, the operation is said to have handled wagers totaling KRW 14b (US $12m) per day and the gang reportedly earned profits of KRW 30b ($25.6m) since 2011.