the signal was somehow blocked by rugged underwater terrain or other obstacles. >> even plant life, seaweed, plant life, coral, any of those things can detract from the signal and even temperature changes in the water, if you have a thermal layer, it can hide that pinging. it can hide any kind of detection that you might have. >> reporter: oceanographer david gallo helped find the recorders in the air france crash with the help of sonar, high-resolution cameras and deep-sea drones. >> we took 85,000 still images of the wreck site, handed those over to a company phoenix international and they took it from there and found the black box boxes. >> reporter: if the boxes are found investigators can still retrieve the vital information inside. >> the battery running out will not preclude the authorities from being able to extract that data. it is embedded into the system. >> reporter: athena jones, cnn,