This is really sad news. I grew up watching SpongeBob right when it premiered in 1999. Little did I know it would be the massive cultural phenomenon it is today.
In my opinion, Hillenburg the 21st Century's Walt Disney, its Walter Lantz (of Woody Woodpecker fame), its Max Fleischer (Popeye, Betty Boop) as a matter of fact. He was definitely a creative genius, and his work on SpongeBob will definitely live on forever.