This is complete bullshit, but its pretty freaky.
There is some story about russian scientists who dug a hole 14.4 km into the earth. This is true I believe. The drill bit went spinning really fast, they had reached hollow. The lowered microphones to listen for shifts in the earth.
They heard something that scared them so much they stopped the project, and quit in terror.
anyways, here the sound. Its pretty freaky, dont listen unless you dont get scared too easy.
http://www.av1611.org/sound/misc/dighell.ram