Michael Phelps

The greatest medal winner in Olympics history, a burnt-out Phelps called it quits after his fourth Games at London 2012 with 18 golds to his name.

He spent 20 months on the sidelines, gaining weight, playing golf and being treated for alcohol problems before deciding the water was where he wanted to be, returning for the 2016 Rio Olympics.

The American crowned his glittering career with five more golds to extend his all-time Olympic record to 23 gold, three silver and two bronze medals at the age of 31, well beyond the typical peak age for a male swimmer.

He has subsequently revealed a battle with depression since retiring for a second time.

Agence France-Presse