I'll tell you then. It's ancient graves.
I thought of it when I remembered some books I had read. The old Vikings got buried in piles of rock, without any opening. Also, in the Middle Age, some cultures made piles of soil, and buried several people inside them, with an entrance. I just made a combination.
The reason I actually put it into the scenario is that I thought; maybe they did archeological research in the ancient times too? Then, since it is a possibility, I just put it in.
†
Sweet baby maizes!