Recently it came to my attention from a non-publicly posting list member that many so-called mounds around the world have subterranean components to them. This evidences that while many have thought mounds were simply "burial" locations, it is much more likely it was efforts from the previous surface dwellers to seal off cavern world entrance/exit points and may explain why there are so many fewer sightings of such things in regions where there are many mounds. LiDAR and GPR (ground penetrating radar) scans are showing more and more evidence of much more underground activity than one may have traditionally believed - and what the Smithsonian would have wanted you to believe.
Another interesting topic is that there is evidence from some cultures that the bodies of the dead have been used as efforts to stop "supernatural" events or entrances from occurring. One such instance that is perhaps very open and evident is the use of the graveyard in front of the gates where Christ is to return through in Jerusalem. Some believe that the dead or their bodies can seal up such passages and subsequently it may be also why some of these "mounds" may have dead bodies/graves in them as well.
Very interesting food for thought - and further research.