You are in the oceans of North America, 70 million years ago.

The water was a brilliant, sun-dappled turquoise, but was too quiet. The ammonite schools that you have been documenting had vanished minutes ago, leaving you alone in the shallow waters of the Western Interior Seaway.

Then, the shadows changed. At first, you thought it was a reef formation—a deep, jagged ridge rising from the seabed. But the "reef" was moving, gliding with terrifying grace and speed, cutting through the water at an angle that made your blood run cold.

It was a Prognathodon. It was immensely broad, a heavily built, 12-meter (40-foot) marine reptile that made the water around it shudder. The sunlight caught the dark, scaly skin and highlighted its most terrifying feature: a deep, massive, boxy jaw that looked like it could crush a ship's hull, let alone your diving gear.

Its head swung toward you. It didn’t look intelligent; it looked hungry…