The Inuit of Arctic Canada take huge risks to gather mussels in winter. During extreme low tides, they climb beneath the shifting sea ice, but have less than an hour before the water returns.
The 500 people of Kangiqsujuaq, near the Hudson Strait, go to great lengths to add variety to their diet of seal meat, seal meat and yet more seal meat.
This settlement and a neighbouring community on Wakeham Bay are thought to be the only places where people harvest mussels from under the thick blanket of ice that coats the Arctic sea throughout the winter.
The locals can only do this during extreme low tides, when sea ice drops by up to 12m (about 40 feet), opening fissures through which the exposed seabed - and its edible riches - can be glimpsed. The best time to go is when the moon is either full or brand new, as this is when the tide stays out the longest.
Filmed for the BBC's Human Planet, they lower themselves into these temporary caverns to gather as many fat and juicy mussels as they can before the tide rushes back in.
It is a risky operation. The ice above is no longer supported by water, and it shifts and groans ominously during the harvest.
A look-out keeps watch for the returning tide, but warning shouts cannot be too loud in case the echoes bring down the ice.
Then it's a scramble to get out before the shifting ice closes the escape hole and seawater refills the caverns.
"We all know stories of mussel hunters who didn't make it out in time. If you can't get out, you die," Mary Qumaaluk told the Human Planet team. She later died in a quad bike accident.
Mussel gathering is a tradition that goes back generations in Kangiqsujuaq, on Quebec's Ungava Peninsula.
But the locals say it is getting harder to find places safe enough to venture beneath the ice, which freezes later and melts earlier than it did even a few decades ago.
Although very few statistics exists, there is anecdotal evidence that travel in the arctic - by snowmobile especially - has becoming more hazardous as sea ice is thinner year by year.
As a result, dog sledding - in decline for years - is making a comeback.
"Dogs are more reliable in the unpredictable weather," says Human Planet director Nicolas Brown. "They can always find their way home, possessing a virtually infallible internal navigation system that works even in a blizzard. Dogs also can smell thin ice."
And, he adds, a stranded arctic traveller can eat dog meat, if desperate. "More than one of the famous explorers has resorted to that solution."