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

The water in the Cretaceous sea was warmer than expected, but the silence was absolute—until the pressure wave hit you. You were diving near a rocky drop-off, examining a coral-like formation, when the light filtered through the water suddenly vanished. A shadow, vast and dark, passed over you.

It was a Hainosaurus, a mosasaur close to 40 feet in length, navigating the shallow coastal water with terrifying grace.

It didn't look like a snake, as you had imagined, but a massive, sleek predator with smooth, dark scales that seemed to swallow the sunlight. Its shark-like tail powered it forward, leaving hardly a disturbance in the water. You froze behind a pillar of rock, holding your breath, as its giant head turned in your direction.