I think Uchiha Madara was the 3rd Mizukage. This is the reason why the 3rd Mizukage hasn't been revealed. He took over after the 2nd Mizukage died, and ruled over sometime. It's during this time he unlocked Rinnegan. He died mysteriously here, but not before handing over the rule to Tobi, who manipulated the 4th Mizukage. It's fairly obvious that Madara and Tobi are working together, its been this way from the start.
What do you guys think?
What do you guys think?