Michael Phelps
Credit: Rafael Neddermeyer / Getty Images

Why he hung it up: Phelps won 22 medals – 18 gold – across three Olympics, cementing his status (potentially permanently) as the most decorated Olympian in history.

Why he returned: So far, Phelps has been quiet on the subject, but we can only imagine he got tired of golfing and playing poker. 

His Success: We're a long way from Rio 2016, but don't bet against Phelps. He's basically a sea mammal.