An ancient Israelite immersion pool. Before making a sacrifice, the Israelites must reside seven days on the temple mount. On the third day and on the seventh day they are sprinkled with living water mixed with the ashes of one of the nine Red Heifers. On the seventh day they washed their clothes and would then be immersed. On this evening they were then clean and could approach the north side of the altar to make their sacrifice.