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.
You mean evidence like the 100s of hours of unexplainable phenomenon recorded each year (see google search of ufo video 2014). Including that which was release from classified status by an arm of our armed forces which chose to not only investigate, but to classify the investigation.
And for the record, we have more video and imagery of these unexplained phenomenon then we do of the giant squid. They, for whatever reason, just dont leave bodies floating up on the beaches.
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.