Stephen Hillenburg, the visionary creator of "SpongeBob SquarePants" died Monday, Nickelodeon and Jessica Berger, a rep for his family, confirmed to USA TODAY. The cause of death was ALS.

ALS, or amyotrophic lateral sclerosis, is a rare terminal illness that affects nerve cells in the brain and the spinal cord. It's also known as Lou Gehrig's disease.

"SpongeBob" is Nickelodeon's all-time biggest hit, and has aired for nearly two decades since its premiere on May 1, 1999, bringing characters SpongeBob, Patrick, Squidward, Mr. Krabs, Sandy Cheeks, Plankton, Pearl Krabs, Mrs. Puff and Larry the Lobster to life.

"We are sad to share the news of the passing of Stephen Hillenburg, the creator of SpongeBob SquarePants," the children's channel tweeted Tuesday. "Today, we are observing a moment of silence to honor his life and work."

Hillenberg's cheery SpongeBob, awash in puns and happily flipping Krabby Patties at the Krusty Krab eatery, became a pop culture sensation. Famous folks filed in to voice characters over the years, including Amy Poehler, Johnny Depp, LeBron James, Tina Fey, Will Ferrell, Kristin Wiig, Mark Hamill, Betty White and Jon Hamm.

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The franchise extended to the silver screen with "The SpongeBob Movie" in 2004 (which grossed $140 million worldwide), which Hillenburg wrote, produced and directed. It was followed by "Sponge Out of Water" in 2015 ($325 million worldwide), which Hillenburg wrote and executive produced. A third "SpongeBob" film will hit theaters in 2020.

David Hasselhoff, who starred as himself in "The SpongeBob Movie," remembered his time on set.

"Wow what a unique and fantastic character Steve created! It was my pleasure and honor to be in 'SpongeBob The Movie' and to share some great laughs with this gentleman," the "Baywatch" alum tweeted. "To this day I am stopped and flattered by people and kids who have seen me in the film."

The squishy yellow guy also made a move to Broadway. "SpongeBob SquarePants: The Broadway Musical," which opened in December 2017, was rewarded with 12 Tony nominations, though the show closed after less than 10 months.

"The fact that it's undersea and isolated from our world helps the characters maintain their own culture," Hillenburg told the Associated Press in 2001. "The essence of the show is that SpongeBob is an innocent in a world of jaded characters. The rest is absurd packaging."

“We are incredibly saddened by the news that Steve Hillenburg has passed away following a battle with ALS," Nickelodeon's statement said. "He was a beloved friend and longtime creative partner to everyone at Nickelodeon, and our hearts go out to his entire family. Steve imbued 'SpongeBob SquarePants' with a unique sense of humor and innocence that has brought joy to generations of kids and families everywhere. His utterly original characters and the world of Bikini Bottom will long stand as a reminder of the value of optimism, friendship and the limitless power of imagination.”

The television series touched Busy Philipps' family: "Sponge Bob has brought my little one Cricket so much joy and given her the weirdest & wildest sense of humor. Thank you Mr. Hillenburg and rest in peace."

Hillenburg first parlayed his fascination with ocean life into a career as a biology teacher at the Ocean Institute in Dana Point, California. In 1987, he began his career in animation, and from 1993 to 1996 got his start at Nickelodeon on "Rocko’s Modern Life." From there, Hillenburg began work full time, writing producing and directing an animated series that would eventually become "SpongeBob SquarePants."

He revealed his ALS diagnosis in 2017, saying in a statement he intended to keep working on "SpongeBob," which is renewed until 2019.

“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 is survived by Karen Hillenburg, his wife of 20 years, son Clay, mother Nancy Hillenburg (nee Dufour) and brother Brian Kelly Hillenburg, his wife Isabel and nieces Emma and Hazel.

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