The solar eclipse is just a few days away, and many Americans are traveling to the “band of totality” stretching across the United States in time for the Aug. 21 phenomenon. If you’re there, you may want to look out for one of the weirdest mysteries involving solar eclipses.

They’re called “shadow bands,” and they’re a strange phenomenon that sometimes shows up during solar eclipses. It’s visible only when the sun has just a rare of light peeking past the moon. And scientists aren’t really sure how it happens.

They look like strange gray ripples that seem to flit over the ground just minutes from a totality. As totality draws closer, they get more and more visible, and the pattern reverses when totality end.

Scientists have proposed a number of theories. One is that the bands are a diffraction pattern created by waves of light passing through a thin part of a solid surface. Another scientist in 1924 proposed that they were overlapping pinhole images caused by gaps in the Earth’s upper atmosphere. A more probable explanation is that the sun’s rays are being distorted by Earth’s atmosphere, so essentially a meteoroligcal effect.