A pile of rocks. Or a reaction surface.
This model has been cleaned up so that it can be used in OpenSCAD as an imported model. A simple example is included, where the mirroring is removed by slicing the model in half (making it printable without supports).
The author remixed this model.
Decimated to something like 1% of the original number of polygons, merged together, mirrored, then exhaustively cleaned up until CGAL stopped complaining.