It’s midday on Iceland’s Falljökull glacier, and all is still. The landscape is impressive, but simple: broad brushstrokes of ice, rock, and sky.

The wind picks up. But there are no trees with rustling leaves here, no bird to catch an updraft. Not a creature is stirring—except for a mouse. Propelled by a blast of wind, a baseball-sized ball of moss called a glacier mouse scoots across the ice.

From a distance, the glacier mouse brings to mind a fuzzy, dense tumbleweed. But, scientists have learned, it’s really more like a mobile home: Each glacier mouse provides lodging, food, and transportation for hundreds of tiny organisms.

Nothing happens quickly around here, and the birth of a glacier mouse is no exception. It begins with a small pebble, furred with a moustache of moss. As the moss spreads, it insulates the pebble against the cold. The ice beneath it begins to melt. Eventually, the now moss-covered pebble balances on a spire of ice. A gust of wind knocks the newly minted glacier mouse off its pedestal and into motion for the first time.

It was the glacier mouse’s impressive powers of insulation that first led scientists to suspect its busy inner life. Biologist Steve Coulson, of the University Center in Svalbard, and his colleague Nicholas Midgley, of Nottingham Trent University, knew that the Racomitrium moss could absorb water and keep out the cold. Did other organisms know this? There was only one way to find out.

The scientists stuck thermometers in five glacier mice to monitor their internal temperatures and brought 10 others back to the lab. Coulson pulled the moss balls apart, hoping to find a few inhabitants. He found more than a few.

The tally, as Coulson and Midgley reported in the journal Polar Biology, was impressive: 73 bug-like animals called springtails, 200 tardigrades, and 1000 nematode worms—all in a single mouse.

Not only did the researchers find these critters, but they found them at all stages of development, which suggests they’re breeding within. And why wouldn’t they? They’ve managed to find a cozy, well-stocked den. The average temperature on the glacier was about 32°F. But inside the mouse, temperatures stayed toasty, between 36°F and 50°F.

There’s another advantage to life inside the glacier-mouse Catbus: travel. The glacier-hopping moss balls carry their passengers to exciting new locations (which, to a springtail, might mean that lump of ice over there).

Herds of glacier mice are best known in Iceland, but they’ve been spotted on the glaciers of several countries. The next time you find yourself trekking across a vast glacial expanse (you know, whenever), take a look at the scenery around you. If the glacier mice can teach us anything, it’s that even the humblest among us contains multitudes.

Photographs of glacier mouse herds courtesy of Darrel A. Swift, University of Sheffield.