according to gearspedia-
The Jacinto Plateau was a large granite plateau located in the COG nation of Tyrus. The plateau was semi-resistant to Locust Horde digging, and as such was one of the few remaining places of human civilization during the Locust-War. Cities such as Ephyra, Jacinto City, Tollen, Ilima and Montevado were built upon the plateau, and after the passage of the Fortification Act, these cities were transformed into military fortresses, while the remaining sections of Sera were destroyed to prevent them from being used by the Locust. Several portions of the plateau were sunk in 14-15 AE, and the plateau was finally evacuated during the final moments of Operation Hollow Storm.