Well, probably the saddest and most terrifying place in the whole universe is in the center of the earth - a tortuous prison called Hell or Hades. It’s where those who have not received Jesus Christ go after death. Not sure what the hum is, however.
My thoughts as well.
#8 Some are happy to be there.
Maybe there are just plain, material things about Earth that we don’t yet understand; and the “hum” is a product of one or some of those.
I don’t believe in ‘hell’; but if it existed, I don’t think it would be a physical realm rooted in Earth, any more than ‘Heaven’ is.
Get your logic together.