I'm pretty sure UFOs are just us in the far future, building saucer-shaped time machines to travel back in time to warn us now people.
Unfortunately, due to massive future deregulation of safety requirements, plus massive funding cuts to pilot training, navigation, math, and science in general, the future-us time-travelling "aliens" are not able to safely land or even fly their craft.
That's my theory, anyway.
I was working on the flightline at Norton AFB in 1979 or 1980, looked up and spotted a couple squads from that unit practicing fast-roping out of a couple of Hueys from maybe 200 feet up. I can't remember what that unit was called at that time, it was something like "Air Commandos". They were practicing for some big public display later that week. I watched 'em for a few minutes. They were not smooth operators like you see in the movies. One guy got stuck on the rope about 25 feet below his chopper and ended up hanging by his boot for a few minutes while the rest of the team figured out how to get him down.
Thankfully nobody was hurt, but I could feel the whole team's embarrassment from where I was working a few hundred yards away. I imagine there was a lot of yelling involved afterwards.