A gyroid filling a cube, 4 Pi on a side: this is 8 single cells joined together in a larger cube.. The formula (a trigonometric approximation to the true minimal surface) is: Cos[x] Sin[y] + Cos[y] Sin[z] + Cos[z] Sin[x] = 0. This surface appears in nature, frequently in oil/water surfaces where the oil separates the water into two domains, at the microscopic level. In cells, it appears in the endoplasmic reticulum. In some butterflies, it is the structure of the iridescent scales (where the holes are the right size to cause light interference like a diffraction grating). This was plotted in Mathematica, solidified in MeshLab.