In 1993 D. Eigler and his collaborators created a so-called "quantum corral" by placing 48 iron atoms in a circle on a copper surface and thus demonstrated a shining example of forming nano-objects through the manipulation of individual atoms with the use of scanning tunnelling microscopy (STM). A quantum corral acts as a two-dimensional cylindrical quantum well confining surface state electrons. Circular waves observed inside the corral in the STM image are standing waves of electron density, whose existence is predicted by Schrodinger's equation for those boundary conditions. The possibility of building quantum corrals of various shapes (triangular, rectangular, hexagonal, etc.) was shown. Using an elliptical ring of cobalt atoms on a copper surface, the researchers demonstrated the effect of "quantum mirage": when a cobalt atom was placed at one focus of the ellipse, a mirage of the atom appeared in the STM image at the other focus. Moreover, the electronic properties of the two-dimensional electron gas surrounding both foci were similar, even though the cobalt atom was only present at one focus.