Neither did we ... but it's true -- and what's even cooler is the way Phelps spent the money.

"10 years ago after winning 8 golds in Beijing I received a million dollar bonus," Phelps said on Instagram.

"There was one thing and one thing only that I wanted to do with that bonus. I started the [Michael Phelps Foundation]."

Phelps says the foundation was initially created to teach the importance of water safety and how to live healthy and active lifestyles.

But since then, Phelps has dealt with substance abuse and mental health issues -- and says he's drawing from his own experience to help others.

"Since becoming aware of the importance of my own mental health we have worked hard to add a mental health awareness component. I’m incredibly proud of how far my foundation has come, but there’s still more work to be done!!!"