Every human eyeball has exactly one spot in each eye where you can’t see. This is because sight requires light to enter your eye and strike the light sensitive cells on the back of the interior of your eye. But your eye doesn’t grow any light sensitive cells on the one part of the back of your eye where the optic nerve connects, so any light that hits that spot isn’t registered or seen.

Sighted people don’t tend to notice these blindspots for two reasons. First, if you can see in two eyes, the two different blindspots don’t overlap, so one eye can see the small region that the other can’t. Second, the human brain has a tendency to fill in the blind spot with color from the surrounding area so that you don’t have a hovering black spot in the corner of your vision.

If something is entirely within the blindspot, however, the brain doesn’t know its there and you can’t see it. You can test this by taking a piece of paper with three small dots on it and slowly moving it back and forth in front of your face with one eye open. You’ll want to look at the center dot and you might eventually notice one of the two side dots vanish out of your peripheral vision.