good evening, everyone. i m alisyn camerota. welcome to cnn tonight. we re learning tonight the navy picked up the sound of an implosion on an underwater learning system on sunday, just hours after the sub lost communications. why did the search go on for four more days? we ve got a live report from the pentagon coming up. we ll break down everything we know tonight about the victims on the ill-fated journey and what went wrong. plus, you just heard christiane amanpour s exclusive interview with former president obama. tonight we ve got reaction to what he says about the indictments of donald trump. we ll also examine what he says about his former vice president, joe biden. stick around for that. let s begin with news of the titanic submersible. cnn s jason carroll is live in boston where some of the coast guard searchers were based and orrin liverman is at the pentagon for us. you re getting new information about this top secret at the time navy system that detected