What test must Gilgamesh pass to get the help of the man scorpion?

Gilgamesh has embarked upon a dangerous quest. Still grief-stricken over Enkidu's death he's desperate to find Utnapishtim, the sole survivor of the apocalyptic flood that deluged the earth. Gilgamesh wants to learn from Utnapishtim how he too might escape death.
But before any of this can happen, Gilgamesh has to go past the twin-headed mountain of Mashu, which is guarded by two fearsome giant scorpion monsters, one male, the other female. When Gilgamesh shows up, they are astonished to see a mere mortal show such boldness in approaching the gates of Mashu. Nonetheless, they are also impressed by Gilgamesh's great courage and sense of purpose and so agree to talk with him.
The Scorpion-man tells Gilgamesh that Utnapishtim lives on the other side of the mountain. In order to get there, Gilgamesh will have to walk through a long tunnel buried deep into the mountain. The tunnel is pitch-black, and the Scorpion-man doesn't believe that any mortal, not even one as brave as Gilgamesh, will be able to survive such darkness.
However, Gilgamesh is undeterred and begs the Scorpion-man and his wife to allow him to make the attempt, which they do. So Gilgamesh enters the tunnel, along which he has to walk in complete darkness for the next twenty-four hours.