There are so many clues of "most likely" visitations of Earth by extraterrestials going back thousands of years indicate that this planet has been a place of great interest for intelligent life elsewhere.
Nazca Lines, Native American art on cave and canyon walls, and the stories passed down generation-to-generation in just about every civilization on Earth regarding not only sightings but ET contact are compelling. The Zuni tribe (NE Arizona) believe their ancestors were brought to Earth by alien spacecrafts and were told to "colonize" Earth. The costumes they wear for their tribal dances on "holy" days have "helmet" looking head dresses that resemble colorful astronaut helmets, sometimes with tubes (resembling oxygen hoses) that go from the helmet and into the cloth covering the body. The Zuni dancers outfits indeed "look" like some sort of astronaut outfit. The same outfit can be seen on cave and wall art that scientists believe to be 4,500+ years old. Donagan and "Chariots of The Gods" take a story from the Old Testament that hints that Elisha wittnessed a UFO and then was taken up inside of one.
Going to mufon.com, it's relatively easy to look at some of the most recent sightings and figure out that what was seen was explainable and not a UFO. A couple are clearly meteorites. A couple are easily either LEO's (Low Earth Orbit) satellites, and others can either be the Hubble Space Telescope or the international space station. Others are not so easy to dismiss.
A week ago a few ex-Air Force officers came out and clearly stated that US nuclear missile silo sites in the Dakotas have been visited regularly by differently shaped UFOs...the missles were "disabled" and then mysteriously re-enabled after the UFOs left. The USAF thought is seriously enough that they scrambled jet interceptors to the scene(s) on multiple occasions and viewed the UFOs as a possible threat to national security (and hense, the demanded secrecy).
There is still so much about our own planet we just really don't know. Ball lightning, for one, has only been recently filmed in the UK. There have been ocean species thought extinct for millions of years show up in fishing nets. For all we think we know about dry land we still don't know jack about whats at the very depths of our own oceans. Think about it, if your UFO could withstand the vacuum of space and the uber-depths of the oceans wouldn't the ocean be a great place to hide?
Some people wouldn't believe that UFO's exist even if one hovered over their house, yanked them up in a light beam and then probed them.
I'd love to see one. (They can probe somebody else, though).