I was introduced to Mithra in History 101 in college about 1958. It was explained as descending from Persian Zoroastrianism and having been brought, in various forms, to Rome...probably by slaves... where gradually, Mithra became the focus until it was displaced by, as DR. J. Kelly Sowards called it "dockyards Christianity".

Here is a quote from the Zoroastrian Avesta scriptures: " We sacrifice to Mithra, The Lord of all countries, Whom Ahura Mazda created the most glorious, Of the Supernatural Yazads. So may there come to us for Aid, Both Mithra and Ahura, the Two Exalted Ones,...."
Rome has a myriad of barely explored underground tunnels and tombs. Some may never be excavated due to the city above them.