In our cover story, David Pogue rides a submersible to the wreck of RMS Titanic. Also: John Blackstone sits down with Grateful Dead co-founder Bob Weir; David Martin profiles a wounded Ukraine veteran who travels to the U.S. for a prosthetic leg; Seth Doane talks with Italian tenor Andrea Bocelli, who had recorded a Christmas album with his children; Martha Teichner examines how mass shootings are being memorialized; Alina Cho visits New York's famed Hotel Chelsea; and Allison Aubrey meets a chef and a vegetable breeder who set out to create a better-tasting squash.