the skull of Arnon found under the mizbeiach

For those learning Yerushalmi Yomi, an upcoming gemara discusses the mistake of King Chizkiyahu in trying to add an extra month after Nisan had already started in order to avoid a pesach done b’tumah. What is remarkable is the source of this tumah – the gemara says that the skull of Arnon haYevusi was discovered under the mizbeiach. How could it be that such pains were taken to insure that the mikdash was tahor, and right under the mizbeiach was the skull of Arnon?! Fortunately for Yerushalmi learners, this occurrence is mentioned by Tosfos Sanhedrin 12a, where we have the Margolyas haYam to help us. Among the explanations he cites is a Chasam Sofer who explains that the tunnels which ran under the mikdash were adjacent to Arnon’s family’s burial plots. Perhaps when cleaning or construction work was done on these tunnels the digging broke through to the graves. The Minchas Eluzar suggests that the skull was not a human head, but was an idol of Arnon which had been buried there, and the concern was for the tumah of avodah zarah. There are other details that need to be worked out – e.g. we usually assume a non-Jew is not metamei b’ohel, and Arnon was presumably not Jewish – so it is worth looking at the Margolyas haYam.