Natural Trap Cave is a limestone sinkhole in the Wyoming mountains. Its opening is a hole about 15 feet wide at the top, leading down 80-ish feet to a cave about 120 feet wide. This makes it almost impossible for an animal that falls in to (a) survive the fall or, if it does, (b) climb out. There is a pile of bones about 30 feet high at the bottom, a rich trove for paleontologists. Water drains out, so it doesn't fill up.
Nice site if you need some natural necromantic energy, or a place sacred to the the morally defensible side of death powers. Throne room of the Vulture King? Or maybe you just need a few hundred cubic feet of ancient (and some extinct) animal bones in a hurry, from rabbits to cheetahs to mammoths.