The hill of bones

Afar, you can see a hill much unlike the others surrounding it. It seems spotted with chalk or some other white stones. As you get closer, you can see that the white spots are in fact bones, hundred and thousands of  bones, broken and scattered throughout the hill. They have been there for quite some time, as the soil has covered some of them and a sickly yellow grass has grown. If you decide to dig, you find that the entire hill is made of bones. If one takes the time, the bones can be recognized at belonging to many species, including humans. What could possibly be at the bottom of that mound?