In 2009, a satellite circled Earth, scanning and sorting the wavelengths reflecting off the planet’s surface. Researchers noticed something baffling: an unexpected wavelength of unknown origin. They tried looking at Earth with only this wavelength, and saw the planet covered in a red hue of varying intensity. So, what was going on? Zoe Pierrat explores the science of chlorophyll fluorescence. Lesson by Zoe Pierrat, directed by Denis Chapon, The Animation Workshop.