Muscovite mica is a widely used material because of its transparency, heat resistance and especially its flatness. This study investigates how flat muscovite mica really is. The surface of cleaved muscovite mica was studied with the help of several optical techniques, surface X-ray diffraction and with atomic force microscopy. The results show that for high-quality muscovite mica, large (> 1 cm2) step-free surface areas exist, which makes it one of the flattest materials around. Several reasons are given to explain why this crystal is so incredibly flat. The flatness of muscovite mica can be exploited in applications such as the surface force apparatus and creating a flat interface for organic solar cells.