EXCLUSIVE: At its upfront presentation last month, National Geographic unveiled Deepsea Challenge, a 3-D feature documentary/special chronicling director James Cameron recent one-man dive last month to the Mariana Trench’s Challenger Deep, the ocean’s deepest point. Now the network has added a half-hour interview special with Cameron, James Cameron: Voyage to the Bottom of the Earth, which will premiere this Sunday at 9 PM. In the special, which incorporates CGI animation to illustrate the scale of the trip, Cameron recalls the highs and lows of the more than seven-year design phase of the spherical sub (called Deepsea Challenger). “I was watching the numbers going deeper,” Cameron says in the special about the expedition, a joined project between the director and NatGeo where he serves as explorer-in-residence. “The sub slows down as you get to the target depth.  There is a long moment of getting to think about it.  Then you have to get busy.  You have less than a thousand feet from the bottom, you fine-tune the ballast, adjust the camera, turn up the spotlight. As the altimeter counted, I saw the glow of the bottom!”