A limestone sink‐hole in the mountains of Puerto Rico has been transformed by a team of stateside scientists into a bowl‐shaped bed for a large‐aperture radio telescope with a reflecting spherical dish measuring one thousand feet from rim to rim. The new antenna, designed to be used both as a highly sensitive receiver for studies in radio astronomy and as a powerful radar scanning device for ionospheric research and radar astronomy, was dedicated on November 1. The surface of the dish, covering an area of more than eighteen acres, consists of a reflecting grid of wire mesh lining a natural concave depression in the hills some twelve miles from the northwestern seacoast town of Arecibo. The project represents a four‐year, nine‐million‐dollar cooperative effort by Cornell University, the Advanced Research Projects Agency, the Air Force Cambridge Research Laboratories, and the Army Corps of Engineers.