A 3D fractal built by subdividing a cube into 27 sub-cubes and removing the center of each face and the center cube — leaving 20. Repeat recursively. Dimension = log(20)/log(3) ≈ 2.727.
Drag to rotate. At depth 3: 8,000 cubes remain from the original 27,000. Surface area grows without bound; volume → 0. The Menger sponge is homeomorphic to the "universal curve" — every 1D compact metric space embeds in it.