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_