The site of Pompey's Pillar and Serapeum is a strange place, and a disappointing experience if you go there without absorbing a little bit about its history. Originally, here stood the main temple of Serapis, the Serapeum. The temple was situated on top of a rocky outcrop, and had 100 steps in front of it. The temple was also a library of religious texts, and had three subterranean galleries where the Apis bull resided. The pillar, with its circumference of 9 metres and height of 25 metres, was added to the complex in 293 CE, in honour of Diocletian. At some time in history, the idea that it belonged to Pompey arose, but there is no connection.
The temple was destroyed by fanatical Christians in 391 CE, and they did a thorough job. The great pillar, however survived, together with a few sphinxes.
A circumferance of nine metres seems a little w..i..d..e
There's a recently-published book out (available at Borders) detailing the search for Alexander's sarcophagus, listing clues from history and making guesses as to where he might be buried. Sorry, don't remember the title, but it looked interesting.