Before Swedish slow TV hit “The Great Moose Migration” began airing Tuesday, Ulla Malmgren stocked up on coffee and prepared meals so she doesn’t miss a moment of the 20-day, 24-hour event.

"The Great Moose Migration" tracks the animals as they swim across the Angerman River and make their annual journey toward greener, summer pastures. This year's 24-hour programme from SVT Play ...