Short answers, pulled from the story.
The first known monument appears around the year 101 CE and was dedicated by an official named Alcimus. This artifact marks the sudden appearance of underground temples called mithraea across the Roman Empire during the last quarter of the 1st century CE.
Archaeologists have cataloged over 420 sites containing materials related to this cult yet no written narratives or theology from the religion itself survive for modern study. It has been estimated that there would have been at least 680 mithraea in the city of Rome alone.
Seven grades of initiation existed including Corax Raven Nymphus Bridegroom Miles Soldier Leo Lion Perses Persian Heliodromus Sun Runner and Pater Father. Admission into the community was completed with a handshake with the pater just as Mithras and Sol shook hands.
Mithraea are commonly located close to springs or streams where fresh water appears required for some Mithraic rituals. These temples are sunk below ground windowless and very distinctive often converted from natural caves or basement levels of apartment blocks.
Franz Cumont published a two volume collection of source texts and images in French between 1894 and 1900 arguing the Roman religion was the Roman form of Mazdaism disseminated from the East. John R. Hinnells and R.L. Gordon criticized this theory at the First International Congress of Mithraic Studies held in 1971 stating it received no support from Iranian material.