Dating back to the first century AD, scientists, philosophers, and other observers have noted the occasional occurrence of "Bright Nights," when an unexplained glow in the night sky lets observers see distant mountains, read newspapers, or check their watches. Few, if any, people observe Bright Nights anymore due to widespread light pollution, but new findings show that they can be detected by scientists and may still be noticeable in remote areas. The new study suggests that waves in the upper atmosphere converge over specific locations on Earth and amplify naturally occurring airglow -- a faint light in the night sky that often appears green due to the activities of atoms of oxygen in the high atmosphere. Normally, people don't notice airglow, but on Bright Nights it can become visible to the naked eye, producing the unexplained glow detailed in historical observations.