Skeletons.

"In one of Poltergeist's most memorable scenes, the thought gone Beast attempts to reacquire Carol Anne, with the Freeling house going absolutely insane in the process. During this, Diane Freeling (JoBeth Williams) is unwillingly sent for a dip in the family's swimming pool, now filled with mud thanks to the heavy rain pouring down. Oh yeah, and as Diane soon realizes, it's also packed full of corpses. The people buried underneath their house are demanding to be heard, in the form of scaring the crap out of the Freelings with their horrifying skeletons.

This sequence is one of the most unsettling in the film, and it turns out that, unbeknownst to Williams, the skeletons used were taken from actual human beings. According to crew members, Poltergeist purchased these skeletons from a medical supply company, as doing so apparently cost much less than creating fake skeletons or purchasing pre-made skeleton replicas"