[I]t seems that the [Altaic] shaman makes vertical descent down the seven successive "levels," or subterranean regions, called pudak, "obstacles." He is accompanied by his ancestors and his helping spirits. As each "obstacle" is passed, he sees a new subterannean epiphany; the word black recurs in almost every verse. At the second "obstacle" he apparently hears metallic sounds; at the fifth, waves and the wind whistling; finally, at the seventh, where the nine subterranean rivers have their mouths, he sees Erlik Khan's palace, built of stone and black clay and defended in every direction. The shaman utters a long prayer to Erlik (in which he also mentions Bai Ülgän, "him above"), then he returns to the yurt and tells the audience the results of his journey.
Mircea Eliade, Shamanism. Archaic Techniques of Ecstasy, trans. Willard R. Trask, Bollingen Series LXXVI, Princeton University Press, Princeton and Oxford, 1972, 2nd ed. 2004, pp. 200-201.