Also I’m watching this right now. Before they go into the cave one of the women starts talking about all the dangers of caving. She mentions auditory and visual hallucinations, paranoia, panic attacks, disorientation. Sounds like what the characters went through.
Great movie. It’s a long time since I saw it, so I can’t remember all the details, but I felt something like that was hinted at. The title itself hints at “descent into madness”. But I don’t think we know for sure. Better to leave it unanswered than a soap-opera-esque reveal that it was all in her mind?