In the far north of the Roman Empire , where the climate was harsh and survival uncertain, a group of soldiers lit lamps in the darkness of an underground temple. Before them, the face of a god carved in stone gazed back. Nearly 2,000 years later, that altar reappeared in Scotland, changing what we know about life on the frontier of the Roman Empire. In 2010, archaeologists in Inveresk—surveying the site for a cricket pavilion—made an unexpected discovery. They unearthed two altars that date back to around A.D. 140, when the territory was reoccupied under Emperor Antoninus Pius. One altar was dedicated to the sun god Sol, a symbol of light, order and cosmic power; the other was consecrated to Mithras, a deity associated with salvation and protection. Notably, this is the first evidence of a cult of Mithras worship in Scotland, the northernmost point of Rome's expanse. “On the northernmost frontier, you need to have the gods on your side, because it’s an uncertain time and an uncertain area.…