[BigV]

It's spinning very quickly.

The researchers found that once the plate was spinning so fast that the water span out to the sides, creating a hole of air in the middle, the dry patch wasn't circular as might be expected. Instead it evolved, as the bucket's spin sped up, from an ellipse to a three-sided star, to a square, a pentagon, and, at the highest speeds investigated, a hexagon.

...

Similar polygonal shapes have been reported in gigantic, vortex-like flows in the atmosphere of our planet and others, as well as in the eye of a hurricane. And an immense, hexagonal-shaped vortex was spotted by the Voyager spacecraft at the northern pole of the gas-giant planet Saturn.

No word on the skulls, though.