Issa Lopez's horror roots are shining through and it gives me joy. So much creepiness and fun jump scares.
That said, my theory is that the answer will be "scientific". I think the mine has poisoned or tainted the water supply and that is why people are seeing things and hearing things. Mass hallucinations would even explain the odd behavior in the animals, like the caribou freaking out and jumping to their deaths in the opening scenes. The dogs of Navarro's boyfriend acting up. The polar bear wandering into town, assuming that wasn't a hallucination on Danvers' part because her late son had the one-eyed stuffed polar bear toy.
It would be twisted if the only real murder was Annie K's and done to shut her up because she was protesting the mine. The researchers deaths are related to the poisoning from the mine. Perhaps Clarke was also figuring it out, hence his message on the board at the research station "We are all dead".
But I also think the show will leave open the possibility of something supernatural or spiritual going on. There's a very "woman of science" (Danvers) vs "woman of faith" (Navarro) element.
