I believe a lot of what people see has some scientific explanation. I remember seeing a story once about paranormal happenings and this lady who saw all sorts of wierd lights in her bedroom. After finding out she wasn’t on drugs, scientists were able to induce the phenomena in a lab using radio signals that interfered with brain patterns. When they went back to her house, they scanned all over her house for a source of such radiation and found nothing, until they saw a clock radio. They measured the electromagnetic or radio waves (I forget) and it had a similar pattern to what was seen in the lab.
U.F.O’s has such a broad definition. You could live in a hole for 20 years, not tuned into anything going on in the world, and when you came out you would see an aircraft that you couldn’t identify. If your talking about spacecraft, could be a balloon reflecting light, expecially if it has many areas of silver on it.
Aliens- if your talking about our universe alone, then its quite possible that we are the only ones here. There may be some super-universal law that nobody knows about that states that only one organism can be dominant in a universe. If there are multiple universes then it is highly unlikely we are alone in the humongous scheme of things.
Bermuda Triangle- Many experiments have been performed, and the leading theory is that a massive release of methane gas can cause the phenomena experienced here.
Of all above, UFO’s are probably most believable.