America Airlines tweeted out a photo on Thursday of a “mysterious” phenomenon that continues to spark debate and baffle some as to its exact cause.

The picture is of what’s called a “pilot’s halo” or simply “glory.” It’s when a rainbow-like image seemingly engulfs the shadow of the plane:

Now, while it’s generally accepted that “glories” are formed by water droplets in the air, there is still plenty of debate about the finer details. One very dense mathematical website sums it up: “…it is perplexing that the glory cannot be explained (even by eminent scientists) except by resorting to mathematical formulations that offer little insight into the mechanisms that actually cause the glory.”

However, there is another site that brings together plenty of the explanations floating around.

Either way, enjoy the beauty, because the conditions have to be just right for it to be visible.