You are in the tropical oceans of Australia, 468 million years ago.

The Ordovician waters here was thick and warm. You were hovering just above the seafloor, watching a cluster of rugose corals, when the light changed, a sudden shadow killing the sunlight. Above you, drifting like a nightmare made of polished bone and glass, was an 18-foot straight shell—an Endoceras.

Its massive shell was perfectly conical, an armored cone of crimson brown and green banding drifting lazily. But it was the front that froze your blood. From the cavernous opening of the shell, dozens of writhing, pale tentacles reached out, probing the water for any sign of motion. You held your breath, hidden behind a clump of crinoids, as the eye—nearly the size of a lemon—seemed to gaze directly toward your hiding spot.

The creature drifts towards you with eerie silence, using powerful, silent jets of water to steer its massive length, hunting for prey in the Ordovician shallow sea.