Well, the "underground" claims are because there's VAST tunnel systems and confirmed areas with massive structures underground that are all barred off to the public. Some for military reasons, some for safety reasons, some for cultural/historic reasons, etc... It's doubtful the government has blocked off public access to all these tunnel systems and underground cities just for them to collect dirt... So many people speculate they are in use for something and "rich people farming hot blonde girls/boys for sex slaves" seems like a reasonable use of these systems by our evil governments.