As the team explores, we learn that there is a strange parasite native to the underground ecosystem of the caves, one that transformed the original survey team into the very monsters attacking the current expedition.
In the film's final scene, two of the three remaining survivors — Katheryn and diver Tyler McCallister (Eddie Cibrian) — share a quiet moment after their return to civilization. Tyler wonders if his brother Jack (Cole Hauser), who sacrificed himself so they could escape, might still be alive down there, while Katheryn says that she initially thought the parasite was native to the underground caves and couldn't survive outside of them. Now she believes that isn't true, and that the parasite wanted to get out. She leans in to kiss Tyler on the cheek, and as she does so, we can see past her sunglasses and learn she has the same telltale warping of the irises that Jack had during his infection. Katheryn walks off into the crowded city streets with Tyler giving chase, but he loses her in the throng, helpless in the knowledge that not only is she infected but will likely infect others as well.
