Here it is
https://eclipse2017.nasa.gov/exploring-shadow-bands
Exploring Shadow Bands
These mysterious bands of shadow race across the landscape in the seconds before totality. Scientists still don’t fully understand what they are. You can study them yourselves by taking measurements and photographs and come up with your own hypothesis.
Shadow bands are thin wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse. Shadow bands have been noted throughout history.
In the 9th century CE, shadow bands during a total solar eclipse are described for the first time-in the Völuspá, part of the old Icelandic poetic edda. Hermann Goldschmidt of Germany notes shadow bands in 1820 visible just before and after totality at some eclipses. George B. Airy, the English astronomer royal, saw his first total eclipse of the sun in 1842. He recalled shadow bands as one of the highlights: "As the totality approached, a strange fluctuation of light was seen upon the walls and the ground, so striking that in some places children ran after it and tried to catch it with their hands"