| Misidentified objects and sensor effects | Balloons, satellites, aircraft, astronomical objects, or camera or radar effects look unfamiliar or seem to move unusually. |
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| Secret human technology | Classified aircraft, drones, surveillance systems, or electronic warfare belonging to the US, China, Russia, or another country. |
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| Unknown natural phenomena | Atmospheric or other physical processes that we haven't yet identified or understood. |
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| Extraterrestrial visitors | Some UAPs are spacecraft from other planets, operated by aliens or sent as autonomous probes. |
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| Interdimensional beings | They come from another dimension or reality, which supporters say could explain sudden appearances and disappearances. |
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| Hidden intelligence on Earth | The cryptoterrestrial hypothesis: an unknown civilisation lives here, perhaps underwater or underground. Some versions propose ancient visitors who stayed. |
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| Future humans | UAPs are time machines, with our descendants returning to observe or study the past. |
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