The medieval ship lay more than a half-mile down at the bottom of the Black Sea, its masts, timbers and planking undisturbed in the darkness for seven or eight centuries. Lack of oxygen in the icy depths had ruled out the usual riot of creatures that feast on sunken wood.This fall, a team of explorers lowered a robot on a long tether, lit up the wreck with bright lights and took thousands of high-resolution photos. A computer then merged the images into a detailed portrait.