Under the square in front of the Qubbat al-Khayyatin are a series of large underground vaulted chambers, some of which are still walled-off and not fully explored.[1] The structure is popularly described as a "prison" where Christian captives were kept, with one story claiming it was built and named after a Portuguese prisoner who Moulay Isma'il promised to release if he built a prison that could house more than 40,000 captives.[37] However, most scholars believe the structure was in fact a storehouse or silo from Moulay Isma'il's time.[3][4][4]:111 The number of Christian prisoners was far smaller (by some estimates, less than a thousand) and they were likely housed in several different areas at different times; the last of which was near a Franciscan convent in the old medina, near the present-day Zaytuna Mosque.[5]:123 The subterranean structure today is nonetheless still known as the Qara Prison or Habs Qara (Arabic: حبس قارا‎, lit. 'Prison of the Bald').[37] Other parts of Moulay Isma'il's palace complex were apparently also built on similar vaulted substructures which served as storage for supplies and military equipment.[6]:396