
The feature is approximately 25,000 kilometers across and extends at least 100 kilometers deep into the atmosphere.
The hexagonal feature was originally detected during NASA's Voyager flybys of the early 1980's, so has apparently persisted for at least a few decades.
No similar feature exists in the south polar region, nor as far as we know on any other planet.
http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=735