Footage showing the phenomenon has been spread around the Internet in the last few days, and you can easily see why.
The footage comes from a PBS documentary on Vikings, specifically a segment on attempts to hunt reindeer.
When startled, reindeer sometimes band together and keep moving in a constant swirling motion, in a behavior that is thought to confuse predators and make it more difficult to pick out a single animal from the herd.
"It's a defense strategy, designed to confuse any predator," PBS said in the documentary. "Whether wolf, bear, or Viking – achieving a clean kill in a swirling melee like this is difficult."
Similar defensive formations have been observed in fish, as well as forming into a large ball or tornado-like schools, scaring off even large predators.