The battle for Echo Base near the beginning of The Empire Strikes Back is one of the most iconic set pieces in the history of science fiction cinema. It perfectly sets the tone of the conflict to come, with the tiny, fragile Rebel airspeeders flitting chaotically around the lumbering, invincible Imperial Walkers, forced to sacrifice themselves and employ unorthodox tactics to buy their allies time to flee the ice planet of Hoth. Visually, it’s a mesmerizing sequence, and the Imperial AT-AT Walkers, with their bizarre, four-legged design, reminiscent of ancient war elephants, are the stars of the show.

Art director Joe Johnston was one of the people responsible for bringing these behemoths to life, using a variant on stop-motion animation developed by effects artists at Industrial Light & Magic. Johnston has released some old Super 8 footage of the stop-motion animation process on his YouTube channel, showing the massive work required for a simple shot of the Walkers making their way across the icy tundra. It’s shocking to see just how much energy went into the sequence, with effects artists meticulously adjusting at least half a dozen articulation points on each of the three visible models for each of dozens of frames.

In the minds of people who saw Empire for the first time as kids, the AT-ATs loom tall, completely convincing as implacable, unstoppable—barring a well-deployed tow-line or two—engines of death. That enduring image is a testament to the hard work of the effects artists to make these implausible contraptions walk with the heavy grace of something that really could stride slowly across the surface of a distant world.

And here’s footage from the film, showing the Walkers in action.