A mini black hole roughly the mass of a thousand sedans at a size that is smaller than an atom. At that size the black hole wouldn't swallow much matter and would instead mostly trap atoms and some larger molecules into circling orbits in much the same way that protons in atoms capture and bind electrons. These mini black holes show up all around Earth every day, but they’re so little that no one can see them and they aren’t causing damage, so they aren’t worrisome.
They work much differently than large black holes do in space. Scientists are guessing the black holes began popping up very soon after the universe was born into existence. Smaller black holes are thought to actually be losing mass and eventually evaporate. The mini black holes are way too small to devour anything. So, scientists continue to study them, but there are no concerns about these microscopic black holes.