Imagine stepping into a life-sized whale carcass decoy and steering it into deep water. You’re looking — yes, looking — for a group of hungry sharks to spark a feeding frenzy. To attract them, you shoot out hundreds of gallons of synthetic blood and chum. Then watch them lose it. That’s what marine biologist Liv Dixon did for Discovery Channel’s “Shark Week,” one of several eye-popping moments during the 21 hours of new programing this year where scientists risk everything to understand the apex predators better. The week, hosted by John Cena, kicks off Sunday with Dixon’s hour-long “Belly of the Beast: Bigger & Bloodier.”