This was the said tunnel featured in Tunnel (터널) (OCN, 2017) that started 80s detective Park Gwang-ho’s (Choi Jin-hyuk) time-travelling ordeal to the present day.
Gujinpo Tunnel (구진포폐터널) is located in Naju, Jeollanam-do.
West entry: 35.004003, 126.675580 (towards the woods)
East entry: 35.004508, 126.677621 (towards the main road)
— submitted by Mich KDL