Part of the problem I've always had with UFO's is the way that "they" are always able to remain on the periphery of observation, evidence, or proof.
Many have seen them, but descriptions are as varied as can be. Saucers, cigar shapes, blue lights, green lights, strobes, electronic dampening effects, proximity alterations in conciousness (my favourite), pyschic emanations, small ones, big ones, little pink saucers with glowing polkadots.....one account I once read described a vehicle that strangely reminded me of the architecture of most "Federation" starships....
My point being, is that ET seems to have this fabulous ability to appear to a few, in isolated circumstances, or even occasionally before many, but never in a totally obvious and profound way, like hovering over the Superbowl for thirty seconds so that 50 million viewers can go........!!!????? bringing about global awareness that "they" are here.
This of course, shares in a modern way the very traits of traditonal folklore, wherein our ancestors saw Leprechauns, dragons, sea monsters, angels, various classical as well as regional gods and demi-gods, and of course, any other magical figures popular within the context of any given historical culture.
I've known a few navy guys who have served on subs, and that's a line of work where they are always looking for undersea vehicles, of course, this being mainly of the human kind. None of them have any stories of strange undersea cities or unexplainable sensor contacts that weren't whales or Russian/Soviet subs.
I doubt if a secret civilization either under the ice throughout our polar regions or under our oceans could mask their existence within the scrutiny of our modern military technological culture. As well, they would not be immune to accidents. And forget about coverups. I don't care how many papers I've signed or oaths I've made. If I had actual knowledge of UFO/ET's even within the military sector there is no way I could keep it to myself over time. It's just too big, too fantastic.
After all this time and interest, the evidence is all too flimsy, or what one would expect to exist from the imaginings, hallucinations, and yes, frauds, of human thought and fascination.