sources close to the operation. they say the bluefin had been calibrated to go to a certain depth. it s not clear why but the vehicle started to move towards a lower depth and at that point it took itself out of the mission. a source close to the operation tells cnn, the bluefin-21 is back in the water, searching for possible debris in the same place it was looking monday night. the source says the underwater drone is operating at about the same depth as its first mission, even though in that first operation it overextended. the one condition that causes it to abort its dive is if it reaches its operating depth of 4500 meters. and that s what happened in this case. reporter: the bluefin was programmed to go a certain depth but it fell slightly below that level, down towards 2.8 miles below the surface. that s its maximum depth. it s unclear why it went so low but when it did, its safety mechanisms kicked in and brought itself up to the surface, 7 1/2