The spring is located directly under the tomb of Sheikh Adi, which is the next room going onwards. To the right of Shekh Adi's cenotaph are stairs leading down to Zimzim spring, which is forbidden to non-Yezidis.
Two openings a little beyond [from the entrance into the main nave], lead into as many rooms, in each of which is a tomb, similar to that already described; and in one a door to some subterranean apartment, which we were not allowed to enter. [Late described in a floorpan as a chamber from where sacred clay is brought.] Badger 1852, p 106-107; of trips 1843-1850
Badger, George Percy. 1852. The Nestorians and their Rituals. Google Books