In soundtrack one there are ghostly voices which speak to the character as he walks past the holes in the ground. It is implied that these holes lead to somewhere in the underworld and the voices are lost souls telling him to turn back as he will meet the same demise as they have had. These voices were recorded my whispering into a microphone very close to the capsule, and pitch shifting them down. This instantly created an ominous sounding character under the ground, but was quite bassy, to combat this I added an EQ8 to the sound to reduce the amount of low end rumble. Shown below.
Here is the link to hear the sound effect in isolation: https://soundcloud.com/svrdophobia/ghostly-voices