Mithraeum
Under the church there is a 2nd-century mithraeum, related to the presence of the barracks of Roman soldiers in the neighbourhood. The cult of Mithras was especially popular among soldiers. The remains of Castra Peregrinorum, the barracks of the peregrini, officials detached for special service to the capital from the provincial armies, were found right under Santo Stefano Rotondo. The mithraeum belonged to Castra Peregrinorum but it was probably also attended by the soldiers of Cohors V Vigilum, whose barracks stood nearby on the other side of Via della Navicella.
The mithraeum is currently being excavated. The remains of the Roman military barracks (from the Severan Age) and the mithraeum under the church remain closed to the public. A coloured marble bas-relief, "Mithras slaying the bull" from the 3rd century is today in Museo Nazionale Romano.
Read more about this topic: Santo Stefano Rotondo