Cao Cao Yunbing Road is the earliest and largest underground military plank road found so far. Cao Cao's strong and powerful "snow" stands at the stack entrance. The ancient tunnel has one-way, parallel double road, upper and lower roads, and the command has complete tricks. Cat ear holes, traps, barrier walls, tripping plate slots, air vents, Password holes, lamps and everything, no wonder it is the "Great Wall underground".