Aliens, it would almost be statistically impossible for them to NOT exist somewhere in the universe. Aliens advanced enough to come here? Interstellar travel is SUCH an ENORMOUSLY difficult undertaking that automatically and by definition, any alien race capable of it is so advanced, that we could do nothing against them if they were hostile, they could just take it without having to, say, make deals with our leaders, and if they were interested in observation and/or blending in,,,,, they would be so good at it, that it would never seriously occur to us that they were visiting us. They'd be mind-bogglingly good at covering their tracks, better than any Area 51/X-Files (MIB etc...) would have us believe. Therefore, I don't know what UFO sighters/abductees are seeing, but I know what they are NOT seeing, namely, Aliens. So, no, I don't believe in aliens having visited Earth. Also, human civilization has only existed for 10,000 years, has only had a radio presence for 115 odd years, and most radio signals would have been muffled by distance.