CIMRM 895, 896, 897 - Mithraeum and cliff-face tauroctony relief. Bourg-Saint-Andéol, France.
There are many more details in Cumont, vol 2, #279 (p.401-2). This specifies that the Acts of S. Andeol may be found in the Acta Sanctorum 1st May, vol. 1, p.38. Andeol was arrested, according to the account, by Septimus Severus who ordered him imprisoned and said, "Quaerite locum tenebrosum et squallidum, ubi nullum lumen appareat, ibique eum recludite... Tunc unus de militibus, Cerecius nomine, dixit ad Caesarum: Domine, est in alia ripa fluvii coeptum fieri templum invictissimi Martis, sub ipso est crypta constructa, in qua si iubet Magnitudo tua, tutissime poterit iste recludi ... Jubente ergo Severo ductus est homo Dei et in crypta daemonibus dedita impie trusus." ("Find a dark and squalid place, where no light enters, and imprison him there ... Then one of the soldiers, named Cerecius, said to Caesar, 'Lord, there is in another valley of the river a temple begun to be made of the most invincible Mars, under which a crypt has been made, in which, if it pleases your Magnitude, he can be safely imprisoned ... Rejoicing therefore the man of God was led by Severus and held in the crypt impiously given over to demons.") On p.26 the Bollandist editors comment that "haec crypta sub templo Martis exstructa antiquitatem non maximam sapit: Quia Christianorum et quidem iam pacate agentium fuit, non gentilium, aedificandis templis cryptas substruere." (i.e. pagans didn't build underground temples). Cumont comments that this supposed anachronism is easily explained by supposing that it was a confusion of MARTIS and MITHRAE, and that, supposing this, this would date the Mithraeum to the start of the 3rd century.
This is very erudite of Cumont, but how likely is it that the Acta of St Andeol are that ancient?