You are in the oceans of North America, 84 million years ago.

The water of the Western Interior Seaway was remarkably clear, though the sunlight barely pierced down to 50 feet. You hovered, watching a school of ancient, silvery fish shimmering in the deep blue.

Then, the shadows changed.

A sleek, roughly 15-foot form glided from the darkness below, its dark-scaled crimson back nearly invisible against the depths. It was a Platecarpus. It wasn't the giant Tylosaurus, but this "flat wrist" mosasaur was terrifyingly agile. Unlike the slow, eel-like movements of older depictions, this creature was propelled by a powerful, shark-like tail.

The mosasaur turned its massive head, its large, curious eyes tracking you for a fraction of a second, before it accelerated instantly, swimming past you and moving at nearly 25 miles per hour. You were glad that you were far too big to be a snack.

The Platecarous set its sight on the school of fish. It didn't stalk; it launched a sudden, precise ambush. Its long, narrow jaws—lined with sharp, conical teeth—snapped open.