The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Foundation.
The observatory's main instrument was the Arecibo Telescope, a 305 m spherical reflector dish built into a natural sinkhole, with a cable-mount steerable receiver and several radar transmitters for emitting signals mounted 150 m above the dish. Completed in 1963, it was the world's largest single-aperture telescope for 53 years, surpassed in July 2016 by the Five-hundred-meter Aperture Spherical Telescope in China.
Following two breaks in cables supporting the receiver platform in the prior months, the NSF stated on November 19, 2020 that it was decommissioning the telescope due to safety concerns. On December 1, 2020 the main telescope collapsed before controlled demolition could be conducted. NASA conducted a full investigation and reported the findings in Reference "Arecibo Observatory Auxiliary M4N Socket Termination Failure Investigation".