As far as mysteries of nature go, ball lightning is one of the more perplexing. It seems there are as many potential explanations as there are sightings, but in spite of decades of intense interest, none stand out as a clear winner. One of the weirder hypotheses claims these glowing balls are nothing more than light trapped inside a sphere of thin air. A new paper has added fresh details to the proposal, setting physical parameters on what such a light bubble might be like. For centuries, people have recorded accounts of grape-fruit-sized orbs of light moving slowly a short distance above the ground, often in the middle of an electrical storm, persisting for maybe 10 seconds or so before silently winking out of existence.