In the Yucatan, Phil and Jess investigate watery caverns believed to be portals to the Maya underworld. Some say angry spirits are to blame for strange lights and bizarre animal behavior, but the sacred cenotes hide secrets far darker than any legend.

Phil and Jess search whitewater rapids and a frigid Alaskan lake for a legendary beast that's now the target of serious scientific inquiry. Armed with new eyewitness accounts and advanced underwater robotics, the team takes a deep dive for answers.