Algorithmically eroded virtual terrain, 0.2mm detail, bottom hollowed out to reduce volume, wall thickness: 2mm. On the underside is the reverse of the terrain: with lobed hilltops and steep valleys.
Made with:
erode master16ba_400.pgm -oh -oolast -ec 100 -er 1.e-3 -sm 0.3 -vs 0.4