The underground ruins are a set of ruins that were built sometime during the Dark Ages. It consists of two levels with monsters scouring around and a final room where a device that can be used with the Gospel was discovered.
During the Ghost Incidents in Liberl, the ruins and the building above served as the main base for Enforcer No. X: Bleublanc, who used the device to send projections of himself all over Ruan. It was abandoned once he had completed his experiments.
It should also be noted that the ruins are similar to the one at Thors Military Academy in Erebonia. However, without the presence of an Awakener, it is difficult to tell if these ruins are capable of changing its form like the ones at Thors.