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Arecibo
Arecibo determined Mercury's rotation period, mapped asteroids that could strike Earth, and in 1974 produced the first discovery of a binary pulsar — which became one of the strongest indirect proofs of gravitational waves and won the Nobel Prize in 1993. The telescope collapsed in December 2020 after its support cables failed. It is also one of the few examples of major research infrastructure built in an American territory whose residents cannot vote in presidential elections.
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