The underwater search for wreckage from Malaysia Airlines Flight MH370 came to a sudden halt on Tuesday when the robotic mini-submarine Bluefin-21 exceeded its maximum depth and returned to the surface. Bluefin-21 had begun its search on Monday near an area of sea bed known as the Zenith Plateau, around 1,800km north west of Perth, Australia, and close to where ultrasonic "pings" likely to have come from "black box" flight recorders were detected. But after scanning the sea bed for six hours, a sudden…