Adjacent to the Chamber of Avtinas above the Mayim Gate was a mikveh that was used by the Kohen Gadol for the first of his immersions on Yom Kippur. This mikveh was visible from the Azarah so a linen sheet would be held up in front of it for privacy while the Kohen Gadol immersed.In order for a mikveh to be halachically valid it must be filled either with rainwater or with flowing water from a river or spring. Much of the water used in the Beis Hamikdash, including the water for this mikveh, came from the Eitam Spring located outside of Yerushalayim. Aqueducts carried the water to Har Habayis and from there a system of underground lead pipes and clay pipes mounted in niches along the walls brought the water to specific locations within the Azarah. The diagram below shows a cross-section of the Beis Hamikdash as viewed from the south and the path of the water as it flowed to the mikveh. As the water reached the Mayim Gate, some of it was directed up to the mikveh in what was an ingenious feat of engineering that seemed almost magical to those who did not understand how it worked. Fortunately, we are told how it worked: the Eitam Spring was at the same elevation as the top of the mikveh — [slightly more than] 23 amos above the floor of the Azarah (the Gemara arrives at this value by adding the 20 amos of the height of the Mayim Gate and the 3 amos for the height of a standard mikveh). By employing the concept of a siphon, the water could be made to flow against the pull of gravity and up into the mikveh.