A paper just came out with an interesting thesis: What would happen if the Earth were hit by a tiny meteoroid moving at near the speed of light? And by tiny, they mean a millimeter (like a grain of sand) up to 10 centimeters in size. That's because… it's possible hypervelocity meteoroids like this actually exist.
No one's ever detected one, let me assure you. But physically, it's not a completely ridiculous idea. As they point to in the paper, tiny micron-sized dust grains can be accelerated to relativistic speeds (that is, moving close enough to the speed of light that the theory of General Relativity becomes important) in supernova explosions; the fiercely intense light of an exploding star is sufficient to kick them hard enough to get them moving close to science fiction-like speeds.
Extrapolating this up to macroscopic sizes, they speculate that, given how often supernovae go off in the galaxy, tiny meteors moving at very nearly the speed of light could be hitting the Earth on the order of once per month. If true, they could be detectable via various means, including infrasound microphones and an array of light detectors spread across the Earth.
Could this be ...