The best theory I've come up with so far, after brushing up on von Daniken's "Chariots of the Gods," is that it's the Hex Nut of the Giants, affixed to the end of a massive bolt that's holding the planet together. I haven't worked out yet how a race of titanic engineers managed to insert the bolt at Saturn's south pole. Nor have I identified the location of their hardware store, but we need to start looking for it right away, because NASA's video shows that it's swirling counterclockwise dangerously near what looks to me like the end of the bolt. If this thing keeps unscrewing ...

This atmospheric feature, first spotted in 1980, turns out to be more than a transient gap in the clouds. Last year it was spotted again by the the Cassini spacecraft, whose infrared spectrometer captured the first full image of the entire hexagon. Here's NASA's summary of expert opinion:

"This is a very strange feature, lying in a precise geometric fashion with six nearly equally straight sides," said Kevin Baines, atmospheric expert and member of Cassini's visual and infrared mapping spectrometer team at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We've never seen anything like this on any other planet. Indeed, Saturn's thick atmosphere where circularly-shaped waves and convective cells dominate is perhaps the last place you'd expect to see such a six-sided geometric figure, yet there it is."

The hexagon is similar to Earth's polar vortex, which has winds blowing in a circular pattern around the polar region. On Saturn, the vortex has a hexagonal rather than circular shape. The hexagon is nearly 25,000 kilometers (15,000 miles) across. Nearly four Earths could fit inside it.

The new images taken in thermal-infrared light show the hexagon extends much deeper down into the atmosphere than previously expected, some 100 kilometers (60 miles) below the cloud tops. A system of clouds lies within the hexagon. The clouds appear to be whipping around the hexagon like cars on a racetrack. "It's amazing to see such striking differences on opposite ends of Saturn’s poles," said Bob Brown, team leader of the Cassini visual and infrared mapping spectrometer, University of Arizona, Tucson. "At the south pole we have what appears to be a hurricane with a giant eye, and at the north pole of Saturn we have this geometric feature, which is completely different."