In Iceland they have this delicacy called hákarl that recently initiated diners describe as “the worst tasting food on Earth,” “the world’s foulest food,” and “the worst thing I have ever had in my mouth.” To say it smells like a urinal would be generous. Not that anyone should be surprised, considering hákarl is rotten shark meat fermented in the dirt or open air for months on end.

Hákarl is no ordinary delicacy, but then again, it comes from no ordinary fish: The Greenland shark has toxic flesh (hence the detoxification via fermentation). It's also one of the most mysterious, weirdest, and largest sharks on Earth. It dives thousands of feet deep in arctic waters and grows to over 20 feet long. It’s also comically slow, averaging less than a single mile per hour—yet, bafflingly, it seems to be an apex predator.

For the Greenland shark, pretty much everything is on the menu. Surveys of their stomach contents have revealed squid, fish, and whale meat. In 2013, two dudes in Canada found a beached Greenland shark that may have been choking on a piece of, um, moose.

Gregory Skomal

The Greenland shark is a scavenger, and it certainly has the teeth for it. “The upper jaw has sharply pointed teeth, almost like needles, and those are really well adapted for sinking into flesh and holding onto it,” says marine biologist Gregory Skomal of Massachusetts Marine Fisheries. “The lower jaw has teeth arranged in rows that look very similar to what one would see on a saw used to cut wood, and so they're really nice cutting tools.” The Greenland shark is probably getting a grip on a carcass with its upper teeth, then twisting to gouge out a chunk of flesh with the lower teeth.

But that mouth could also do the shark well for hunting live prey. Consider the cookiecutter shark, which has similar dentition. This small, zippy species gouges flesh like the Greenland, only it targets living fish and whales and at least one unfortunate man on a swim between Hawaiian islands—at night (I mean, I'm not his father, but come on). The Greenland may be doing the same to marine mammals in the arctic. Scientists have photographed beluga whales, for instance, with big plugs of flesh taken out of them.

Then we have the curious happenings on Sable Island off the coast of Nova Scotia. Seals here have shown up with corkscrew-shaped wounds spiraling down their flesh, at times with half their bodies stripped of skin and blubber, like peeled potatoes. Some scientists say it could be the work of Greenland sharks, others blame boat propellers. No one has ever caught a shark in the act, so the culprit remains a mystery.

“I have personally been in the water with Greenland sharks and handled them like they were inanimate objects,” Skomal says. “They're incredibly docile and don't appear to have any capacity to accelerate and capture a seal.” But consider the fact that seals can sleep in the water, bobbing with just their heads above the surface (known as “bottling,” because why not), to avoid polar bears. That’d make them vulnerable to Greenland sharks.

Lazy, But at Least in a Productive Way

If you ask biologist Peter Bushnell of Indiana University, South Bend, the seals may not even need to be sleeping for the Greenland shark to snag them. Given the presence of so much fish in the shark's stomach contents—and often a total lack of mud, which you’d expect to find in something that’s just scavenging on the seafloor—he has no doubts it's an active predator.

When a sheet of ice covers the sea, seals pop in and out of holes to hunt fish—a behavior the Greenland sharks may be exploiting. “I have a feeling they can slowly meander their way up to an ice hole and just park themselves for a couple of days,” Bushnell speculates. “And a naive seal plops itself in and there it is.” Thus could a lethargic predator manage to take down a spry victim like a seal while barely exerting itself.

Live fish, too, may be food for the Greenland shark, Bushnell reckons. The shark’s jaws are somewhat extensible, and like a lot of fish (oh, and a certain 6-foot-long salamander), the Greenland may be able to rapidly open its maw to create a suction effect. Imagine the shark sneaking up on a school of cod in the deep dark sea: "With a little bit of a lunge forward and a suck, you've got dinner,” Bushnell says.

What makes this even more incredible? A good number of Greenland sharks can’t see worth a damn, thanks to a parasitic crustacean that bores into their eyeballs and feeds on their corneas because there is no justice in the world. It’s all the rage, really: One study found that 100 percent of Greenland sharks caught near Svalbard, Norway had parasites attached to their peepers.

The parasite can severely damage the eye, either impairing the shark’s vision or snatching it away altogether. But by virtue of being a shark, the Greenland can fall back on its fantastic sense of smell—and indeed it has a massive olfactory bulb in its brain.

Gregory Skomal

Yet if Greenland sharks are indeed going after slumbering seals, or ones that drop into the water right in front of their faces, maybe vision isn’t all that important. On the flipside, though, the Greenland’s “big nose is really effective at finding scent trails from dead animals,” says Skomal, “and therefore supports the notion that it's evolved to be a deep-water scavenger.”

Clearly, the ecology of the Greenland shark is still somewhat of a riddle. The active predation part is a particular head-scratcher. “It is a bit of a conundrum, but the one thing that is clear is they are active predators,” Bushnell says. “How they do this, I’m not sure.”

What is also clear is that the Greenland shark is well-adapted for the deep, and this may be where its toxicity comes into play.

Greenland Shark Meat: It’s Like Kibbles 'n Bits, Except It Gives Dogs Explosive Diarrhea

Back in the ‘60s, soon-to-be-regretful humans fed Greenland shark meat to a pack of sled dogs, which suffered convulsions, respiratory distress, and explosive diarrhea. The problem may have been high levels of the compound trimethylamine N-oxide (TMAO) in Greenland shark flesh. This could help the animal maintain osmotic balance—that is, balancing its internal salt chemistry with the salt chemistry of the water. But why would levels of TMAO be so much higher in a Greenland’s flesh than in other sharks?

It may, Skomal reckons, come down to the uniqueness of the arctic environment, and especially the deep-sea arctic environment: lots of salt. “You tend to have higher salinity areas in northern latitudes,” he says, “because you get this very thick seasonal ice layer and sometimes permanent ice layer that extracts fresh water from the ocean.”

When the sea freezes, fresh water forms the ice, while the salt is left behind. This boosts the salinity of the remaining flowing water in the area. But because saltier water is heavier, it sinks to the depths, while the less salty stuff goes to the top. So as a scavenger in the arctic depths, the Greenland shark would do well to have higher levels of TMAO to help it maintain that balance between its internal salt chemistry and the salty deep waters it frequents.

That said, the Greenland shark is still largely a mystery and a source of some contention among scientists. Does it hunt? Sure seems like it. Does it scavenge? Yeah. Should you trust eating its flesh if it’s been processed and detoxified? That all depends on how good your health insurance is.

Sorry, Icelanders, but sometimes we Americans have to give in to caution.

Browse the full Absurd Creature of the Week archive here. Know of an animal you want me to write about? Are you a scientist studying a bizarre creature? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.