“They may already be here, well established in safe underground bases”.
You are close, but still with the perspective of physical and in the same realm as us. And still with limitations based on our own limited knowledge. They have always been here since our time began. But not in our physical dimension. There is a parallel dimension or more that they have mastered beyond our own. They can jump between the dimensions at will and manipulate time and space as they do so. So location and time are not linear or static for these entities as it is for us. Same concept would be the perspective that along with our physical world everywhere are also the nonphysical dimensions of heaven and hell permeating everywhere simultaneously.
I never said that they were ETs. I don't know what the phenomena is, but I'm absolutely sure that anybody who says they definitely do know is FOS.