When you spend your life in the water, I guess you tend to develop a good intuition for its subtleties. Dolphins have been observed to create bubble rings by exhaling air carefully in the middle of the vortices caused by the motion of their fins through the water, among other techniques.
Besides being nice to look at (and a neat demonstration of fluid mechanics), this phenomenon also might throw some light on dolphin cognition, since the skill to create the rings is a bit subtle and tends to be taught from one dolphin to the next via careful observation and practice. I'm also intrigued by the report that they seem to be using sonar to locate the vortex in the water, since that would be a fairly amazing bit of audio analysis.
This video is dedicated to the best employee at Sea World.
See more here: http://www.earthtrust.org/delringgallery.html