People are talking about hippos this week, at least in part because the Cincinnati Zoo’s beloved baby hippopotamus, Fiona, is now three months old—a milestone that seemed uncertain when she was born prematurely in January.

Fiona’s doing great—so great that she’s “a little bit dangerous to actually cuddle and snuggle” anymore, the zookeeper Jenna Wingate told local reporters.

Which reminds me that wee Fiona will eventually, if she continues to thrive, turn into a grown-up hippo. And grown-up hippos are not—I repeat, not—to be trifled with. Consider, for example, this video, which my colleague Ed Yong shared with me yesterday:

Look at that hippopotamus go! After sharing this video on Twitter, I got several perplexing responses. Comments like this: “Not bad for an animal that doesn’t swim,” and “And … they can’t even swim!” There is even, someone told me, a children’s book about this: Hippos Can't Swim: And Other Fun Facts. As a long-time skeptic of “fun facts,” I obviously had to know more.

The San Diego Zoo’s fact sheet about hippos wasn’t exactly clarifying: “hippo limb muscles are for powerful propulsion through water, but not swimming.”

What is swimming, if not using one’s limb muscles for powerful propulsion through the water? “Well, a boat doesn’t swim,” my colleague Molly Ball, pointed out. But hippos don’t have engines, or propellers, or sails. So how, exactly, do they charge through the water so impressively? After contacting half a dozen zoologists and wildlife parks, I finally had my answer.

“Depending on water level they walk or they swim,” said Dagmar Andres-Bruemmer of the Frankfurt Zoological Society. Except the swimming isn’t really swimming per se, she said. Rather, it’s a kind of gallop.

“For all intents and purposes the hippo does not swim,” said Douglas McCauley, an assistant professor in the department of ecology, evolution, and marine biology at the University of California, Santa Barbara. “It almost always maintains some contact with the bottom and walks or bounces off the bottom using these bottom contact points as a source of propulsion.”

This is remarkable for a few reasons. Recall how fast the hippo in the YouTube video seemed to be traveling while submerged. These animals can weigh as much as 10,000 pounds. Their round bodies aren’t exactly streamlined. Yet hippos are able to keep their feet in contact with the ground, even when they’re underwater, “by control of the specific gravity of the body and high bone density,” according to a 2009 paper about the kinetics of underwater hippopotamus motion, published in the Journal of Mammalogy. Also, they’re able to dramatically increase the latitude of their regular walking gait while underwater. “The aquatic environment, although increasing resistance to movement, buoys the animal up,” the paper says. “This increased buoyancy effectively acts to make water a microgravity environment.”

Which means a hippo barreling through the water is often supported on two feet instead of four. It also helps that they can remain submerged for up to 30 minutes. “There are periods in which the hippopotami are ‘in flight’ with no feet in contact with the ground,” the paper says. “In deep water, they locomote by ‘a series of porpoise-like leaps off the bottom’ or in ‘a series of high, prancing steps.’”

Hippos can do all this terrifying prancing because they’ve evolved with just the right combination of buoyancy and bone density to allow it.  

“It is quite lovely to see them do this underwater gallop,” McCauley told me. “They remind me of portly astronauts doing a moon walk underwater.”