Looking up from underwater, one sees the whole sky. But it doesn’t stretch 180 degrees from horizon to horizon like it does above water. Instead it’s compressed into a circle about 97 degrees across, regardless of the observer’s depth. This occurs because light rays bend when entering or exiting water. The shrunken sky seen by submerged observers is called Snell’s Window, informally named for Willebrord Snellius, a Dutch astronomer and mathematician. It’s also called the optical manhole.

From here