Nanotube rings were fabricated from straight single-wall carbon nanotubes (SWNTs) with yields exceeding 50%. The rings result from the folding of nanotubes onto themselves under ultrasonic irradiation to form coils with a narrow distribution of radii (300-400 nm). A simple continuum elastic model is used to discuss the thermodynamic stability of the rings. Their formation involves a balance between tube-tube van der Waals adhesion and the strain energy resulting from the coiling-induced curvature. Our findings suggest that ring formation is a kinetically controlled process where bubble cavitation, generated by ultrasonic irradiation, provides the energy necessary to activate ring formation. The electrical conductance of the rings is measured as a function of temperature and in the presence of a perpendicular magnetic field. While the rings consist of bundles of many metallic and semiconducting SWNTs, they exhibit metallic behavior at low temperatures, and quantum interference effects are clearly observed.