I have no doubt whatsoever that SOMETHING has been visiting our Earth every now and then.

Whether these beings come from another planet, another dimension or whatever, I cannot say for sure.

The Soccoro Encounter/Lonnie Zamora case is a good example of a credible UFO sighting, with solid proof of the UFO's presence being available for evaluation:

http://www.ufocasebook.com/Zamora.html