My theory for the ending *Spoilers*
Having loved the first movie, I decided to watch this one with high hopes.
Thought it was a decent enough movie but definately no where near as good as the first, the re-appearance of Juno was a great twist but up until that point, I thought it was just another basic horror.
So watched it through, watched as Rios finally escaped (in the same kind of shots that we saw Sarah 'escape' with in the first movie ;) ) and all seemed fine until the old guy (can't remember his name so he will be known as OG in this post) hits Rios and leaves her right over the hole for the mutants to get her. It got me thinking back to the first movie.
OG must have been in his late 50's? maybe early 60's, and if you remember from the first movie, the team found old equipment from a previous exploration (I can't remember how old they said it was so it may make my theory invalid) which means that a previous team had been down there (obviously!)
What if the previous team never escaped...except for one...OG!
He may have ventured into those caves years before the first movie, could have gotten seperated and found a way out of those caves and waited for his team to return, but they never did, so he goes back to the cave and finds these mutants, which obviously wouldn't have been there when his team was lost. This gets him thinking and then maybe realising that his team could have evolved into these things (which would explain their Human appearance) and OG would do anything to keep them alive, since a team is not just that, it is a friendship that would have been built up for years (like with Juno and Sarah's team in the first movie) so he will feed them and protect them at any cost, even if it means murder.
I've probably opened up more questions than I've answered with this but what do you all think?