You are in the oceans of North America, 36 million years ago.

The water of the Eocene Tethys Sea was impossibly warm, but a sudden, unnatural shadow sent a jolt of panic through your chest. Below you, perhaps forty feet down, a serpentine shape was moving through the pale green water, undulating like a giant sea serpent—an apex predator that was entirely, definitively a mammal.

Basilosaurus was not just a whale; it was a 60-foot, sinuous monster.

You froze. Its sheer length was overwhelming, looking more like a leviathan-eel than a modern sperm whale. It wasn't diving deep, but gliding close to the surface, its massive, elongated body easily navigating the shallow seas. You could see the sunlight refracting off its dark, dorsal surface, highlighting the weird, elongated vertebrae that gave it such snake-like movement.