The ones seen decades ago.
Ball lighting caused by atmosphere conditions with different minerals in the area more so if rivers or lakes in area.
No such thing as a UFO thousands of satellites have yet to spot one and planes spot blob photos or fake images.
NASA hasn’t spotted on yet
The term UFO just means we don’t know what it is.
I believe that some of these things are truly unexplained, but that doesn’t mean they’re necessarily material aircraft - or even the same things..