Alien UFOs are physically possible, while ball lightning was long thought physically impossible, according to my understanding.
There's been considerable debate in philosophy about what the heck physicalism (the belief that everything is physical) really amounts to, and I (as a physicalist) have tended to think that the real issues may be best described negatively rather than positively; it's not an endorsement of things which sufficiently fit the paradigm of the physical, but rather rejection of what I usually call the spooky. One of the common characteristics of the phenomena I class as spooky is that belief in them is encouraged by well-documented cognitive biases, one of the b...
The Wave: In Pursuit of the Rogues, Freaks, and Giants of the Ocean is a book about very big ocean waves-- the science, the danger (mostly to ships), and the surfers.
Really big waves weren't scientifically verified until about ten years ago-- part of the problem was that even though sailors had been reporting huge waves, scientists had a theory that big waves (maybe over 80', though I don't have a sharp dividing line) required very rare conditions. Once satellite surveillance for waves was possible, it turned out that big waves were fairly common, and might explain why a ship or two per week disappears.
Russell Wynn: "The way the radar system works, the very big ones are difficult to measure," he said. When behemoth waves appeared in the satellite data, the space agencies considered these readings to be errors, and they were automatically deleted. "They give you missing value code instead, which is really annoying. We shout at them for that."
The reason I'm posting this is that I've become very skeptical about any theory which claims that something which is well-attested and physically possible is actually not happening.