The “alien spaceship promoters” use a common optical illusion called the “Crater Illisiuon” The human visual system expects light to come from above, so when it is not, then things look inside out. Craters look like domes.
These shapes are all identical, yet we see some of them as depressions, and some as raised dimples.
So viewed from this angle, the dark sides of the depression look like they are a shadow under a flat disk with
But if viewed so the light is coming from the upper left ... Read even more
