Arctic sea ice likely reached its yearly minimum size on September 19 and September 23 (punctuated by a slight upturn), at its 6th lowest area of coverage in the 39-year satellite record.

This year’s sea ice minimum, reported in a preliminary announcement by the National Snow and Ice Data Center in Boulder, Colorado, is above the dramatic melting seen in 2012, but still well below the average recorded from 1981 to 2010. (The Antarctic, which is close to reaching its annual maximum of ice cover, saw a record low this time last year). Considered signs of Earth’s rising temperature, the seasonal loss of sea ice is worrisome because of the ice’s role in keeping the planet cool. With less ice to bounce sunlight back into space, the oceans warm faster. Warmer oceans, in turn, mean less ice.

It’s easy to forget that the ice is also a world unto itself, says Kevin Arrigo, a biological oceanographer at Stanford University. He’s concerned about changes to the active ecosystems hidden in the ice, and to the ice itself, which forms surprisingly complex and varied structures.

“The biggest misconception is that it’s just too cold and inhospitable and that nothing could possibly live there,” Arrigo says. But while the sea ice hugging the planet’s poles may look like a vast expanse of nothingness, abundant life is found inside and just underneath. Even at its historic record low in 2012, Arctic sea ice formed a habitat with a surface area close to twice the size of Mexico. At its lowest, Antarctic sea ice remains nearly the size of Argentina. Click on the slideshow to explore the hidden worlds in sea ice.

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Scanning electron micrograph of a brine channel within Arctic sea ice
Magnified image of microorganisms living in a sea-ice brine channel
A magnified photo of the alga M. arctica in a chain formation
An icebreaker ship makes its way through Arctic sea ice
Algae cling to the bottom of a thick layer of Arctic sea ice
Sea stars freeze as a brinicle – a tornado of freezing water — extends to the ocean floor in the Arctic
Bird’s-eye view of a ship surrounded by large slabs of pancake ice in the Arctic sea
Melt ponds form on the surface of the ice cover in the Arctic sea
Gif tracking Arctic sea ice seasonal cycles from 2001 to 2016
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Salty waterways A chunk of sea ice looks different from the average ice cube in your freezer. Salty channels wind their way through sea ice, carving out tiny pockets and clouding the ice. Seawater dumps salt, nutrients and other dissolved particles into these spaces in the process of freezing. This concentrated brine is so salty that it stays liquid even as the water around it freezes.

When conditions are right, these pockets fuse, creating flowing tributaries, rich with salt and nutrients, that snake through the ice. Sometimes you can find tiny worms or shrimplike creatures making their way through these channels. But the majority of the life here is single-celled — drifters that get swept into the growing ice from the ocean below and settle into their frozen homes.

SCANNING ELECTRON MICROGRAPH OF BRINE CHANNEL: RACHEL OBBARD

Icy challenges It’s not easy, living in ice. Most earthlings couldn’t do it. But the critters here have found a way to transform themselves — and their homes — to make it work.

Brine dwellers struggle to make do in water much saltier (as much as four times) than waters of the ocean. The lighting isn’t so great either. This poses a challenge for species that live off energy from sunlight and is likely one reason that single-celled species in the ice tend to grow much larger than their open-ocean counterparts, since a bigger surface allows them to harvest more light.

It’s also cold — really, really cold. “Some of these things are living at minus 10, minus 20 [degrees Celsius],” says Arrigo. For comparison, freshwater cubes in your freezer solidify at 0ºC (32º Fahrenheit), while regular seawater usually freezes at around –2ºC.

MICROBES IN ICE: NOAA ARCTIC RESEARCH PROGRAM

Making a home Algae are some of the most abundant life in sea ice and the diatoms — single light-harvesting cells with walls of hard, opalescent silica — are among the most common. Melosira arctica cells are roughly the width of a human hair, but they link together to form chains that can span meters, hanging like drapes from the submerged side of the sea ice. To survive the frigid temperatures, these microbes produce their own antifreeze in the form of a protective slime. Secreted into brine pockets, this mucus not only provides a safe place for M. arctica cells to hide, it can also reshape brine channels, creating more hospitable surroundings.

M. ARCTICA: ALFRED WEGENER INSTITUTE / JULIAN GUTT

The smell of breaking ice Many microbes also survive by producing DMSP (full name: dimethylsulfoniopropionate, in case you’re into long words). This versatile, sulfur-containing molecule protects microbes from the cold, serves as an energy store when sunlight is low and keeps cells from bursting due to osmosis-driven water influx. Microbes ramp their internal DMSP levels up or down in sync with the concentrations of salts outside.

DMSP and its by-products are so abundant in the ice that you can smell them escaping into the air when icebreaking ships come crashing through.

“The stench is remarkable,” says David Thomas, an ocean scientist at Bangor University in Wales, who literally wrote the book (well, one book) on sea ice.

CREDIT: MKEERATI / SHUTTERSTOCK

Growing base Once an ice layer floats atop the ocean, new crystals grow underneath to form thin gill-like sheets extending downward into the sea. Here, the extensive surface area mixed with an ample supply of seawater nutrients creates desirable digs for microbes such as ice algae. Microbes nest between ice chunks or hang in long, mucus-based strands and mats from the ice’s edge, reaching densities higher than those found within the ice. When warm weather comes, their frozen anchors melt and these colonies will fall to the ocean floor, providing food for brittle stars, sea cucumbers and other creatures.

It’s not the only way ice transitions feed ocean life. As salt is ejected from forming crystals, the resulting high-density cold water sinks, stirring up the waters below to keep nutrients circulating.

CREDIT: JILL HEINERTH

Underwater icicles Brine collected within sea ice doesn’t stay put forever. Melting or cracks within the ice can expose a brine tube at any moment, dumping its contents — water, salt and life alike — back into the ocean. Often this happens as brine pockets themselves get cold enough to freeze and expand, cracking existing ice. The escaping brine is dense enough to stream toward the ocean floor, like an underwater river within the warmer and less dense ocean around it. The stream is so cold that it instantly freezes the surrounding seawater, creating a hollow tube of ice that grows as brine pours from its end. These brinicles can sometimes reach the ocean floor, where they creep across the seabed, freezing everything they touch – a kiss of death for bottom dwellers unlucky enough to be in their path.

CREDIT: BBC PHOTO LIBRARY

Pancakes and platelets Brinicles are not the only strange formation in this ever-changing seascape. Slabs, round and flat as pancakes and as large as 10 feet across, arise as the first seawater begins to freeze in autumn. Pancake ice forms when choppy seas break up thin layers of early ice. As the fragments bang together, they form rounded plates with raised rims. Voilà: pancakes! They don’t last for long, though. Eventually they meld into a thickening winter cover.

In the Antarctic, smaller chunks of platelet ice form a longer-lasting habitat. While similar in shape to pancakes, these wispy disks emerge when splashes of supercooled water escape the heavy pressure of an ice shelf. With the sudden release of pressure, they freeze. They rise to rest at the bottom of the ice shelf, forming a nutrient-rich, porous layer of ice on which microbes thrive.

CREDIT: MARIA STENZEL, NATIONAL GEOGRAPHIC CREATIVE

Ponds on the ice While most of the action is happening in and under the ice, the upper face is an important part of this picture as well. In the Arctic, freshwater ponds formed by melting snow decorate the ice cover’s surface. Historically, these nutrient-scarce ponds have done little for ice life. Now, though, as climate change leads to a warmer Arctic, these puddles often melt completely through the ice, merging nutrients from seawater with light from the surface. The combination means much more life in surface melt ponds.

CREDIT: STEFAN HENDRICKS, ALFRED WEGENER INSTITUTE

A changing world Arctic sea ice is always changing. Each summer, the massive expanse shrinks down to a shadow of its winter self, flushing a world of life. With a change in season, the ice rebuilds that ecosystem once again. But, as climate change warms the poles, this cycle is shifting. Already more light penetrates the thinning ice, allowing earlier pulses of microbial life that may be hard on whales, birds and other nomads who time their migrations to Arctic phytoplankton blooms. Another shift is the loss of multiyear ice — the portion of ice that withstands the summer melt. It’s too early to say exactly how these changes will impact Arctic ecosystems over the long term, says Arrigo. What is clear, he says, is that the future will bring a different Arctic.

CREDIT: NASA’S SCIENTIFIC VISUALIZATION STUDIO