Crop circles are not of alien origin, either. The plants spiral (on the molecular level) due to sudden exposure to powerful radiation (from the Van Allen Radiation Belt discharging into a field). The belt is created by the Earth's magnetic field trapping subatomic particles from the solar wind (given off by the suns nuclear reactions). Those particles form discrete boundaries of different types of particles (held apart by magnetism, and sometimes by charge). As the earth moves relative to the sun, the boundaries shift, and (rarely) arc to earth. This creates spectacular patterns in fields, as the tightly bound beam is focused onto a small area (think of a magnifying glass that is tilted, smearing the magnified bright beam from the sun). The Earth's atmosphere focuses the particles.
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So, many bizarre phenomena have perfectly rational explanations, while other phenomena does not.
There could very well be aliens, and they could be the kind to abduct humans (perhaps for scientific study).