It’s a sad day for fans of SpongeBob Squarepants fans. Stephen Hillenburg, best known as the creator of the hit Nickelodeon series, died from ALS Monday (Nov. 26) at the age of 57. Nickelodeon confirmed the sad news of Hillenburg’s passing in a tweet.

“We are sad to share the news of the passing of Stephen Hillenburg, the creator of SpongeBob SquarePants,” Nickelodeon wrote. “Today, we are observing a moment of silence to honor his life and work.”

Hillenburg revealed he had been diagnosed with the neurodegenerative disease known as “ALS” (Amyotrophic Lateral Sclerosis) last year.

“I wanted people to hear directly from me that I have been diagnosed with ALS,” he said at the time. “Anyone who knows me knows that I will continue to work on SpongeBob SquarePants and my other passions for as long as I am able. My family and I are grateful for the outpouring of love and support.”

Hillenburg, who has written and directed for other Nickelodeon shows, taught marine biology at the Orange County Marine Institute before coming up with the idea for Spongebob in the late 1990s. The show’s first episode aired in 1999.

“Obviously, SpongeBob is a comedy, but it really was inspired by me liking marine science,” he said during a 2015 interview about how he came up with the hit show. “I focused on that, and I never thought the two would come together.”

Hillenburg earned nine Emmy nominations for the show and even won a special Emmy Award earlier this year for his animation work.

Though most people know Hillenburg as the creator of SpongeBob, he actually got his start in animation by directing another popular Nick show, Rocko’s Modern Life, and he also wrote and directed for Rugrats.

“Let’s not forget, Stephen Hillenburg got his start on Rocko’s Modern life as a director, producer, writer, and storyboarder,” one fan wrote on Twitter Tuesday. “So we have him to thank for the production of this staple of 90s Nickelodeon as well.”

Director Jorge R. Gutierrez wrote: “A giant of cartoons has left us. A kind, brilliant and hilarious genius who will forever be remembered for his creations. Animation was changed for the better thanks to him. And millions of kids grew up with a character that championed innocence. Gracias for everything, Steve.”

“This year has just absolutely shattered my heart. Lost two creative, imaginative individuals in both Stan Lee and Stephen Hillenburg, who both shaped so much of my own childhood and my childhood wonder,” another fan of Hillenburg’s work wrote.

In addition to the TV series, Hillenburg wrote both hit “SpongeBob” feature films. The first one, released in 2004, grossed more than $140 million at the box office. The 2015 sequel grossed over $325 million.