Temples of Mithras are sunk below ground, windowless, and very distinctive. In cities, the basement of an apartment block might be converted; elsewhere they might be excavated and vaulted over, or converted from a natural cave. Mithraic temples are common in the empire; although unevenly distributed, with considerable numbers found in Rome, Ostia, Numidia, Dalmatia, Britain and along the Rhine/Danube frontier, while being somewhat less common in Greece, Egypt, and Syria. According to Walter Burkert, the secret character of Mithraic rituals meant that Mithraism could only be practiced within a Mithraeum. Some new finds at Tienen show evidence of large-scale feasting and suggest that the mystery religion may not have been as secretive as was generally believed.