You are in the oceans of North America, 80 million years ago.

The water in the Cretaceous interior sea was unusually calm, a murky blue-green expanse under a blistering sun. The air was thick and silent, save for the rhythmic slap of your own body in the diving suit, moving through the water.

Suddenly, a strange, long shadow began to detach itself from the darkness below. It was impossibly long. You first thought it was a giant squid tentacle, but it wasn't undulating. It was stiff, moving with a calculating, slow precision. Only when the small, sharp-toothed head came into view, 15 feet ahead of the main body, did you realize you were looking at the neck of a plesiosaur. A Styxosaurus, to be precise.

The creature didn't move like a shark or a marine turtle. It was stalking, using its incredibly long neck—nearly half of its 38-foot length—to strike at prey without bringing its bulk forward. The small, needle-sharp, conical teeth were exposed, designed for grabbing, not chewing.

For a moment, it watches and circles around you, surveying the strange intruder in its world, not with malice but curiosity.