ELI5 Submarines - how the Titanic was a cover for a US Navy submarine mission

If nuclear submarines can generate their own water and oxygen underwater, why do they need to surface at all? Why are submarine propellers always at the back, while airplane propellers are usually at the front? How did submarines and the US Navy play a part in the discovery of the Titanic? ... we explain like I'm five Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: ilearnrussian, lokiorin, slobbie, hemingwavy, yancy_farnesworth, luckbot, omnipotentsandwich, r3dm0nk. To the community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]