You are in the coal forests of Scotland, 300 million years ago.

The air in the Carboniferous swamp was thick, hot, and smelled intensely of rotting vegetation and ozone. You were stumbling through the knee-deep ferns, tracking a small Petrolacosaurus that had been scurrying in the undergrowth, trying to navigate the dim, humid canopy.

You weren’t looking down.

Your right foot caught on something that felt like a submerged log—hard, segmented, and slightly slick. You plummeted forward, expecting to hit soft mud, but landed instead with a loud thwack on something arched and armored.

You scrambled back, gasping, your breath catching in your throat as the "log" began to move. It wasn’t a log. It was a segment.

Rising out of the mud, easily eight feet long and still continuing, was an Arthropleura. It was like a giant millipede, armored in dark, overlapping dorsal plates that shimmered with an oily sheen. The creature didn’t seem interested in attacking, but your intrusion brought it to a slow halt.

It reared up to size you up—not entirely, as it reached the height of your shoulders, but enough to show you a terrifyingly long, armored underbelly—and turned its head toward you, sensing the air with rapid-fire antennae. You could hear the clicking of its legs against its own armor as it readjusted to face you. You were almost dwarfed by its sheer bulk, a 300-million-year-old titan with dozens of legs, each easily the size of your arm.