There is an aspect of the Dulce case that I find especially intriguing.
That's saying quite a bit when considering a crowded gumbo of strangeness like Dulce.
It is the allegation by several proponents that at least one of the non-human races involved with experiments deep within the base is of terrestrial origin. In other words, they're intelligent, they're not human, but they live here on Earth. I speak specifically of the purported reptilian race (I've also heard them called "reptoids" and "reptilons." Pick your preferred sobriquet.) These beings are said by a few to be a divergent race, split off from the dinosaurs and evolving into bipedal, intelligent hominids (Sleestaks!) They live in the caverns deep in the crust and have thrived there for centuries untold. One area of dense population for them apparently is in or near Dulce. Just what their involvement with the base is or with whom they're allied is still difficult for me to fully discern.
What's more interesting to me is the whole concept of a separate race of intelligent beings sharing the planet with us but never really making themselves known. Could that even be a thing? I doubt it, but if it were to somehow be a reality, I think we're looking in the wrong place by going underground.
Ever considered just how many UFO sightings occur on or near water? Not simply a UFO sighted hovering above the ocean or a lake, but I mean diving into or bolting out from under the sea. Not only that, but there are numerous accounts of underwater UFOs (or USOs) featuring anomalous lights and even solid craft in our oceanic depths. One such case would be the Shag Harbour incident.
Ivan Sanderson was a scientist and an ardent author and researcher in both cryptozoology and ufology. In his book Invisible Residents, he catalogued thousands of cases of unidentified underwater objects, many times being observed by American military personnel or sonar located by submarines. This led him to develop a theory that an entire race of intelligent beings lived in the unexplored ocean depths. In at least one aspect, it makes sense.
We know so very little about our oceans. If you were an extraterrestrial race and wanted to operate on Earth unseen and unbothered (though any reason they'd want that folds under scrutiny), it would be sensible to base your UFOs out of the sea. We seldom go there and we don't really know our way around when we do.
But even more interesting than that is Sanderson's idea of a separate and aquatic race. Maybe they did arise from our own world and have been avoiding us. Can't say that I blame them. While the evidence for such an occurrence is weak, I find the concept fascinating nevertheless.