Mithras was not a god the Romans found at home. He arrived from Persia, or possibly was reinvented in the eastern Mediterranean as something only loosely connected to the Persian original, and his cult spread across the empire from about the first century AD. It was a male-only initiatic religion, secretive, organised in small lodges, with seven grades of initiation and a heavy military membership. Soldiers carried it from one posting to the next. By the third century there were Mithraea from Hadrian's Wall to the Syrian desert. Every temple followed roughly the same plan: a sunken central nave between two raised benches, with an alcove at the far end where a stone relief of Mithras killing the cosmic bull - the tauroctony - stood as the focal point. The Caernarfon Mithraeum had all of these features, in miniature, set 137 metres north-east of the Roman fort of Segontium on the edge of what was then a small marshy valley.

What Boon's team excavated was actually three successive temples, each built on the foundations of the last. Phase I was a rectangular building 14.6 metres by 6.55, dated tentatively to the third century AD when Segontium was garrisoned by the Cohors I Sunicorum - a unit recruited from the Sunici tribe in what is now the Rhineland. The walls were untrimmed beach boulders, the roof was purple Cambrian slate. The narthex, or entrance porch, had been almost entirely destroyed by the 1958 sewer. The only firm dating evidence was a worn denarius of the empress Faustina the Elder (138-141 AD) found on one of the benches - probably an offering. Phase II added timber colonnades, presumably to brace the heavy slate roof, and a v-shaped drain cut diagonally across the floor to collect spring water from a tank near the back. Phase III, after a partial roof collapse, raised the floor with broken slates to fight the persistent damp, added small flights of steps to the benches, and built a stone platform that may have held a statue.

Above the Phase III floor, archaeologists found a thin layer of soil - no more than sixty millimetres - and above that, a layer of burnt debris. The sequence tells a clear story. The temple was abandoned first. The Mithraic sculptures, which would have included the tauroctony and probably several reliefs of Mithras's attendants Cautes and Cautopates, were carefully removed by the worshippers before they left. Soil accumulated over the bare floor for some time. Only later was the building burned - whether deliberately or accidentally is unknown. The timing fits the historical record. The garrison at Segontium was withdrawn around 290 AD as part of the imperial reorganisation of the British frontier. The soldiers took their gods with them. Whatever happened to the Caernarfon Mithras cult after that, it left no trace in the historical record.

The Mithraeum itself is not visible today. After the excavation it was recorded, sampled, and backfilled, partly because the marshy ground that bothered the diggers in 1958 would continue to damage any exposed remains. What you can see is its larger neighbour: the fort of Segontium, partially excavated and consolidated by Cadw, with a footprint visible above ground and a small museum. Walking from one to the other takes about five minutes. The land in between is suburban Caernarfon - bungalows, a school, the same shallow valley that once held the temple stream. Standing in someone's front garden, you are above the foundations of one of the strangest religious experiments of the late Roman Empire. The locals are largely used to it.