this came up repeatedly, a viewer asking, why aren't they using submarines to help in the search? >> that's a good question. the u.s. doesn't talk about what its submarines are doing or what they can do. the reason is, they work at 20,000 feet down, but they only radiate a signal about a mile, so you really have to be on top of it to actually hear it. so the sub may not be the best tool to do that with. but we don't know whether they're actually using them or not. >> to hear it, you're talking about the pinging, we know there are about two weeks later, this is what the sonar equipment would hear, you can't hear it with the human ear, this is what the pinging would hear like with those hydrophones. john, you know that sound well. gena asks -- how long after the original 30 days does the data remain viable inside those black boxes? >> it will be there for many years. because it goes on to a computer chip, as long as that chip is not compromised the data is there.