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style in architecture and design
...was a huge construction. The subterranean basilica near Porta Maggiore in Rome (used by an Orphic or Pythagorean society) was a strong and magnificent structure hidden in a large garden. The Mithraic sanctuaries were artificial caves illuminated from above by light shafts. They were built for communities of 50 to 100 persons.
use in Mesopotamia
...III, the royal family of Adiabene converted to Judaism. In the first two centuries of the Common era, Christianity and various baptismal sects also began to expand into Mesopotamia. So far no Mithraeums (underground temples for the worship of the god Mithra), such as existed in the Roman Empire, have been found in Mesopotamia, except at Dura-Europus, where Roman troops were stationed....
worship of Mithra
The Mithraic sanctuaries were subterranean caverns, which presented obvious limitations of size. None of the many excavated shrines could receive more than a hundred persons, most even fewer. All ceremonies were of necessity enacted in artificial light. The cavern always contained a well. Access to the cavern often consisted of a system of subterranean passages, which were used in the...