The Ending Of What Lies Below Explained

Although it takes a while for his true intentions to reveal themselves, John's behavior is a bit off right out of the gate. When they first meet, he gives Libby a bracelet, the stones and symbols of which he claims are related to ancient fertility rites. It's an odd choice of gift for the teenage daughter of your girlfriend, but he claims he chose it because he knows Libby has an interest in archaeology.

John uses Libby's scientific inclinations to try and bond with her. He tells her that he's part of a research team that's "scattered around the area" that is trying to help increase the adaptability of freshwater animals so they can live in saltwater environments, as global warming is causing an increase in salination. Later, he shows her his basement lab where he breeds lampreys, and even takes her out onto the lake to help him catch some.

During their boat trip, Libby suddenly gets her period. John reaches out to grab a loose lamprey and grazes between her legs. He apologizes profusely for the inappropriate and non-consensual touch, claiming that it was an accident. However, he follows it up by licking the blood off his fingers, which rightfully disturbs Libby.

While she was starting to warm up to him a bit before, after the incident in the boat, Libby becomes extremely weary of John. Those concerns are only escalated further one morning when Michelle, who is extremely ill with what she claims may be morning sickness, asks Libby to get her supplies from the pharmacy in town. While there, Libby sees John canoodling with another woman and wearing clothing that's very different from his usual style. Libby is now certain that something is very wrong.