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

The water of the Western Interior Seaway was warmer than you expected, thick with sunlight and the smell of ancient plankton. You were holding your breath, suspended near the surface, watching the light dance through the deep blue.

Just then, a shadow appeared. It was massive, eclipsing the light from above. At first, you thought it was a mosasaur, but the shape was wrong—too broad, too rounded. As it glided closer, its anatomy became clear. It was a Protostega, a marine turtle larger than any car you had ever seen.

Its shell, or carapace, was not a solid, thick bone like a modern tortoise, but a leathery, mosaic-plated dome that stretched nearly ten feet across. It moved with graceful, powerful strokes of its enormous front flippers, seemingly flying through the water. Its head was massive, ending in a hooked, parrot-like beak designed for crushing crustaceans and mollusks.

It passed barely twenty feet below you. You felt the surge of water as it propelled itself forward. Its dark-brown, leather-like skin was scarred—a testament to life in an ocean teeming with predators.

For a moment, it turned its head and looked at you with a calm, prehistoric eye. There was no fear, only the ancient, slow intelligence of a reptile that had mastered these waters.