A Camera Obscura (Latin for "dark chamber") is the natural optical phenomenon that led to the invention of photography.
When light from a brightly lit scene passes through a small hole into a completely darkened room, the light rays cross. Because light travels in straight lines, the rays from the top of the scene hit the bottom of the wall, and vice versa—projecting a live, full-color, but upside-down image of the outside world.
Before cameras existed, artists used room-sized obscuras to trace landscapes. You can still step inside surviving public camera obscuras today: