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,
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. so that s the good news, they can still get access to that data. but we re 21 days out here and still don t have a confirmed debris field. in the case of the air france flight, the first floating parts will found five days after the accident. those flight recorders were recovered 23 months after the accident. so that gives you a sense of just how long it can take to find these devices underwater. wolf? it can be so, so frustrating. athena, thank you. so we ve seen satellite images that may represent the plane but so far searchers have been unable to find them. brian todd has been looking into this part of the story with an explanation of why it is so hard. wolf, it s one thing to find