As a sibling comment notes most sharks cannot live long in freshwater, and moreover this is soecifically true of Greenland sharks, though they do sometimes spend time in brackish river mouth environments, so, unless it developed the weird behavior of migrating quickly up the relevant underground river to make a quick appearance and then inmediately rushing back down the river to the ocean, that’s one answer we can be fairly certain is wrong.

There are a few sharks that can live in freshwater, but they tend to inhabit warmer oceans.f_

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RajT88 6 hours ago | parent | next [-]

Totally wild factoid: Bull sharks have been caught in tributaries of the Mississippi River in Illinois. (Back before they built all the dams)

	▲	dragonwriter 5 hours ago \| parent \| next [-]
		Yeah, bull sharks are the most common and wide-ranging of the sharks that are adapted to survive in a wide range of salinity levels.
	▲	darkteflon 2 hours ago \| parent \| prev \| next [-]
		Bull sharks are often found in rivers tens of kilometres from the ocean in Queensland.
	▲	cucumber3732842 5 hours ago \| parent \| prev [-]
		That's the kind of rare and highly luck based curiosity they ought to give you a plaque for. "From this shore in 1973 local angler..." Slap it on the same sign post as the flood high water markers they put up.

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joshuaheard 5 hours ago | parent | prev [-]

That's too bad. I thought he was on to something.