Ungoliant's Lair was a ravine located in the southern part of Aman in the clefts of the Pelóri south of the mountain Hyamentir, and west of Avathar. Ungoliant came here shortly after her former master was defeated and imprisoned by the Valar. Here, she lived as a monstrous spider sucking up all light and spinning it into dark nets and webs that strangled all light from entering it, making it one of the most horrifically dark and evil places ever to exist. For thousands of years, she lived, fed, and hid from the Valar whose watchful eyes did not extend into the far south until Melkor came with his offerings.
As Ungoliant went to Middle-earth to live, the lair was presumably abandoned and it is not known what became of it or whether the Valar ever found it and cleansed it of darkness.