While we usually think of black holes as massive, theoretical micro black holes could be smaller than an atom yet have the mass of a mountain. These tiny black holes would warp spacetime intensely in microscopic regions. They are predicted by certain quantum gravity models and might have formed in the early universe.
This matters because micro black holes test the limits of quantum mechanics and general relativity.
It also opens the possibility of detecting tiny black holes through particle collisions or cosmic rays.
Micro black holes could pack a mountain’s mass into a space smaller than an atom.
[ESA, esa.int]