Sometimes improbable but theoretically possible explanations are the closest to the truth

From How Did This Skull End Up All Alone in a Cave in Italy? We Finally Have an Answer by Peter Dockrill.

It was found in 2015 – an isolated clue to a macabre mystery set in motion thousands of years in the past.

This ancient puzzle consisted of just a single piece: a solitary human cranium, discovered all by itself with no other skeletal remains around, resting inside a cave in Bologna, Italy, at the center of a cavernous depression the locals call Dolina dell'Inferno (Hell's sinkhole).

It was not an easy thing to find.

The well-concealed skull, missing its lower jawbone, could only be reached by traversing a difficult cave passage called the Meandro della cattiveria (Maze of Malice), and then ascending a vertical shaft to a height of 12 meters (39 ft), where the cranium rested on a rocky ledge.

Due to difficulty in accessing the spot, speleologists weren't able to retrieve the cranium until 2017, at which point researchers had a chance to study this mysterious, ancient specimen.

The lonesome skull turned out to be ancient indeed, with radiocarbon dating suggesting the cranium belonged to an individual who lived sometime between 3630 and 3380 BCE, placing them within the archaeological context of the early Eneolithic (aka Chalcolithic) period of the region.

Other Eneolithic human remains have been found in the general area; not in Hell's sinkhole, but in a rock shelter approximately 600 meters (nearly 2,000 ft) away from the cave in which the skull was found.

So, the greater context makes some sense. But how exactly did this solitary skull get so far away from its Eneolithic counterparts, positioned high up on a ledge, yet buried within a malicious maze of a cave, and concealed at a depth of 26 metres (85 ft) below the ground?