This week a submersible carrying passengers to the site of the wreck of the Titanic disappeared in the North Atlantic; all five aboard perished in a catastrophic implosion. Correspondent David Pogue, who rode in the same underwater vehicle last year, reports on this latest disaster, and looks back on his interviews with OceanGate CEO Stockton Rush and deep-sea explorer P.H. Nargeolet, who were among those killed.