Had a scary experience in a house I lived in once. It was in the pacific northwest (I won't get any more specific than that for reasons explained later). This was a very, very old house (it was built in the mid to late 1800s), and I (along with some roommates) were renting it. It was two stories tall and had what was originally a dirt basement, but part of it had been filled in, only not all the way (it's sort of hard to describe, so here is an MSPaint image (It's not to scale, this basement was enormous):
The grey parts are the heating vents, which went to each room (they were likely the reason why part of the dirt had been moved, back whenever the heating system was installed). We had just gotten the internet up and running, but this was back when wifi was still prohibitively expensive and nowhere near as commonplace as it is today, so we decided that the best bet would be to run CAT5 down through the vent in the room that the modem was in, running the cable through the hole for the vent, tape it to the outside of the vent, and then run it through the basement and then up the hole for the vent into the other room.
So I'm squeezing my ass through the gap in between the bottom of the house and the top of the dirt, and I notice that a lot of the top has been washed away from a leak in the side of the house (the light blue line). The basement had always been really damp, and flooded a few times, so I assume that is where it came from (the water always went away after a while, but that's a different story). So I am crawling through the dirt, on my back, with a roll of CAT5 in one hand, a pair of pliers and a hammer in the other (there is barely enough room to get through, and it freaked me out a bit because I had to breath shallow or I would get stuck), and a flashlight in my mouth, when I get the bottom of my shirt caught on something.
Turns out it's a belt buckle. I put the cable and the tools down and awkwardly position myself so that I can check it out. I wipe it off and see it's still attached to a belt, so I start to pull on it. I pull it up and I start to see the dirt shift around it. It's attached to pants. I pull more and I see more dirt move and yeah, it's attached to bones.
I freak the fuck out and hit my head and crawl back out as fast as I can and my roommates ask me what the fuck is going on so I tell them, and they don't believe me, so one of them goes up to look and he freaks the fuck out and comes back. So at this point we sort of wonder what to do. I end up calling the Sheriff's Department (instead of the landlord, under the thought process that maybe he had Ted Bundy'd some guy and stashed him there) up and tell them "Hey, I found a dead guy in my basement, what now?", they had me give my address and describe the scene and the body and they rushed out fairly quickly.
Long story short, there were between eight and ten dead people buried under my house, including some children.
The situation is not as grisly as it sounds, though, because they had all been there for over a hundred years. The top layer of dirt had been slowly eroded away by water leaking into the house from the crack in the wall. It turns out that the family who originally built the house were settlers, and once they got there some of them died (as settlers are prone to do), but for some reason they decided it would be better to bury their dead underneath their house instead of in a graveyard or outside (I have no idea why). I also have no idea why they weren't buried underneath the floor of the basement, maybe they ran into bedrock or something. The landlord had no idea either (he wasn't related to the original family, it had been sold quite a few times since then).
So, they ended up being removed, and interred in a cemetery (although they were only able to ID a few of the bodies, one from an inscription on his belt buckle), and the landlord's family and the Sheriff were on pretty good terms so they were able to keep it out of the media (the landlord was planning on selling the house within the next ten years and didn't want that sort of history attached to it) and we ended up moving when our lease was up a few months later. Almost all of them were determined to have died of some sort of natural cause, and I guess a couple of them had broken bones that were really terribly set (frontier medicine), and they could tell by the conditions of the bodies that they had all been there since at or before the turn of the century.
tl;dr - Remember all of those people who died of dysentery, snakebites, and broken arms while you were playing The Oregon Trail? They all ended up in my basement.