You are in the oceans of Canada, North America, 508 million years ago.

The water was hazy, filtering the bright sun of the Cambrian shallow sea through several meters of clear blue. You—donned with specialized diving equipment to handle the current intolerable atmosphere—were hovering just above the seafloor, observing a bustling community of trilobites scouring the algae-covered mud.

Suddenly, the trilobites panicked, scattering in a frenzy of armored legs. A dark, sweeping shadow cast by the sun told you why before you saw it.

Gliding effortlessly with a strange, mesmerizing, undulating motion—similar to a manta ray—was an Anomalocaris. It was massive, easily over a meter long, and looked entirely out of place in this prehistoric world, like an alien entity. Its body structure was clear, with 14 individual segments along its body.

It passed barely a foot from you, its huge, stalked compound eyes—each possessing thousands of lenses—focusing intently on the seabed. You could see the soft swimming flaps along its body working in unison, allowing it to navigate with surprising agility. Its two long, spiny, segmented appendages extended forward, scanning for movement.

As it circled, it revealed the business end: a circular mouth, resembling a pineapple ring with 32 overlapping plates, acting as a camera-shutter of doom. It reached down to grab a smaller trilobite that hadn't moved quickly enough, but the armored creature curled into a tight, impossible-to-penetrate ball. The Anomalocaris paused, perhaps analyzing the prey with its sophisticated eyes, before dismissing it and gliding away in search of easier, softer prey, merely passing you as you watched.