I first heard of ball lightning years ago, and in that case it was described as a rare but well-established natural electrical phenomenon associated with storms, but in round form instead of bolts, and not flickering, with its cause/mechanism not being entirely known. I recently tried checking to see whether more information had become available about it since then, and found out that such a phenomenon isn't verified to even exist, and the name "ball lightning" has a roughly legendary status, due to that and the fact that it's apparently been applied to a variety of things that weren't very much like each other (different sizes, colors, elevations, speeds of movement, durations, amounts of damage done to objects, types of weather they were seen under, and sometimes flamey or having a tail but sometimes not).
The reason I was rethinking ball lightning recently was that I had reason to believe I'd just seen it myself, so now I'm curious what explanations are possible for what I saw. I was outside near the entrance to the store I work at during a storm. Suddenly a ball of bright blue-white light several feet wide appeared above and behind another store across the street and down the block a bit, bright enough to light up the nearby buildings like lightning does. It stayed consistently bright for 2-4 seconds instead of flickering, while floating a few times its own diameter toward my right, and then went out. Electrical service for a couple of blocks around was lost for at least 4 hours.
It wasn't regular lightning, because it wasn't bolt-shaped, didn't flicker, and didn't make thunder. I know there's a transformer back there and I'm sure it popped (which has happened there before), but I've seen another transformer pop before and this was different. A normal one only emits a burst of light for a fraction of a second, and doesn't detach from its pole and go floating around horizontally.
I haven't thought of anything that would behave that way. I thought some kind of gas or a cloud of solid particles could have been released hot and cooled off, but it was perfectly round, and it didn't move vertically, or spread out or change shape, or drift in the wind's direction. I thought of an electrical wire with some kind of energy-releasing reaction passing along it, propagating as a very slow wave, but the light-ball was much wider than a wire.
If it's a not-precisely-explained natural phenomenon that happens in storms, then I guess it falls into what I originally thought was the definition of ball lightning. But even if someone out there knows exactly what it is and it's to be expected sometimes from transformers and power lines in storms, it just looked so odd that I'd bet it's what's behind some "ball lightning" sightings anyway.