Giant squid are in no way "unsubstantiated." Dead ones have been washing up on shores around the globe for centuries. Live ones were elusive until recently, but we now have a lot of footage of those, as well, thanks to things like deep sea oil exploration and various ocean science missions.

If there are aliens, even if they can easily elude us, it seems likely that there would still be traces of them had they visited. Some sort of technological foot prints, whether they be emissions from ships or communications, or actual pieces of technology that have been left behind or broken off. Something, somewhere, you'd think.