I've never doubted the existence of alien life, but UFOs...that's a whole different story. I highly doubt that we've been visited by extraterrestrial beings. I also believe that about 98% of UFO sightings can be explained by natural phenomena/man-made objects. Notice that there's 2% left... I guess part of me wants to believe...

wease:
What are your thoughts?
Things like this:
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Certainly do make one ask the question: "How... the... hell...?" The level of precision many of those stones were carved with was beyond the appreciation of the human eyeball and any known technology at the time. There's also the rule of economics... when these people built this stuff, it wasn't like the whole economy of this place was running simply for the sake of building hideously big structures. Other basic functions of life had to go on, as well.
Look at Cathedrals. Those works -broke- regional churches across Europe and imposed very heavy economic burdens on the population.
These guys built stuff that makes the Cathedrals look quaint by comparison... so it couldn't have been substantially more taxing on their population. In other words... the task of building these structures couldn't have been greater than the task of building a Cathedral.
Which is a mind-boggling concept, considering where we were at the time.
Regardless... we're missing something. Be it aliens or super-smart ancestors that disappeared (or some combination of both...).
As for whether or not many of the strange lights in the sky end up being aliens... I think an exceptionally small percentage of them are plausible candidates for some kind of ET activity. Though little can be gained from what few sightings fall into that category - other than "yeah... that's hard to explain."
Until we catch such a vehicle and coax some answers out of whatever being happens to be inside - I don't think we'll have much of an answer.
Would be very disappointing, however, to expand into the cosmos and have only ourselves to interact with.