"Pompey's Pillar" is the best-known ancient monument still standing today. It is located on Alexandria's ancient acropolis — a modest hill located adjacent to the city's Arab cemetery — and was originally part of a temple colonnade. Including its pedestal, it is 30 m (99 ft) high; the shaft is of polished red granite, roughly three meters in diameter at the base, tapering to two and a half meters at the top. The structure was plundered and demolished in the 4th century when a bishop decreed that Paganism must be eradicated. "Pompey's Pillar" is a misnomer, as it has nothing to do with Pompey, having been erected in 293 for Diocletian, possibly in memory of the rebellion of Domitius Domitianus. Beneath the acropolis itself are the subterranean remains of the Serapeum, where the mysteries of the god Serapis were enacted, and whose carved wall niches are believed to have provided overflow storage space for the ancient Library.