It's that time of year when sharks rule not just the waters, but America's TV screens. Sharks have always been a fan favorite, and two cable TV networks are combining science and research with nonstop thrills and entertainment to teach audiences about these fascinating sea creatures. National Geographic kicked off its ninth annual SHARKFEST programming special earlier this week, which runs for a total of six weeks. Meanwhile, Discovery is in its third decade of Shark Week, with its 33rd annual production launching July 11 and running for seven days.