Introduction to the drainage outlet: There are ventilation holes and wells in the tunnel; there are small pits at intervals in the wall holes, where oil lamps are placed. There are earthen troughs for feeding livestock on the upper floors, and in the middle and lower floors, there are caves for storing food, earthen caves for shelter and large caves for garrisoning troops at intervals. Experts have verified that such a large and complex underground project is definitely not a civilian construction, but an underground military facility built in ancient times according to the "clear fortress and secret passage" in the art of war.