Arecibo Observatory

The Arecibo Observatory sits inside a natural karst sinkhole in northern Puerto Rico and, from 1963 until its dramatic collapse in 2020, housed the William E. Gordon Telescope — at 305 meters in diameter, the world's largest single-dish radio telescope for over five decades. Originally conceived during the Cold War for ionospheric research, it became the nerve center of radio astronomy and the SETI program: in 1974 it transmitted the Arecibo Message, a binary-encoded broadcast aimed at the globular cluster M13, still humanity's most deliberate attempt to contact extraterrestrial intelligence. The dish also gained pop-culture fame through the James Bond film GoldenEye and Carl Sagan's novel Contact. In December 2020 the 900-ton instrument platform fell into the dish after support cables failed, ending the telescope's operational life. The visitor center remains open, documenting the observatory's legacy while the collapsed structure itself stays off-limits.