You are in the oceans of North America, 75 million years ago.

The water of the Western Interior Seaway was warm, murky, and surprisingly shallow. you were skimming just below the surface, your breath holding steady, when the shadows emerged from the gloom below. Not one, but three of them.

Elasmosaurus.

They moved with a haunting, slow-motion grace, completely different from the chaotic darting of modern sharks. Their four flippers propelled them with a rhythmic "underwater flight," a steady, quiet power that made them look more like celestial bodies gliding through space than predators in a sea.

The closest one was about 40 feet long, passing within twenty feet of you. Its neck—a thin, serpentine stalk comprising two-thirds of its length—swayed gently, scanning the water with a small head that held needle-like teeth. You were dwarfed by its presence. The neck seemed impossibly flexible, yet perfectly controlled, a specialized tool for snatching fish without moving its large, turtle-like body.