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Stephen Hillenburg, the creator of the Nickelodeon’s beloved cartoon series, “SpongeBob SquarePants,” died on Monday. He was 57.

Hillenburg revealed last March that he had been diagnosed with ALS.

“We are incredibly saddened by the news that Steve Hillenburg has passed away following a battle with ALS,” Nickelodeon said in a statement. “He was a beloved friend and long-time 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.”

Hillenburg graduated from Humboldt State University in 1984 with a bachelor’s degree in Natural Resource Planning and Interpretation, with an emphasis on marine resources. According to Variety, he became a marine biology teacher at the Orange County Marine Institute (now the Ocean Institute) in Dana Point.

This interest in marine life, combined with his artistic talent, led him to write and illustrate stories as teaching tools with characters that would later become the residents of SpongeBob’s home, Bikini Bottom.

Born in Lawton, Oklahoma and raised in Anaheim, Hillenburg became fascinated with the ocean as a child and developed a love for art. After earning a degree in Experimental Animation at the California Institute of the Arts in Valencia, he went on to work in television as a director and writer on Nickelodeon’s series “Rocko’s Modern Life.”

That’s when he began developing the concept for “SpongeBob.”

“It dawned on me that if I was going to do a show on animals, I’d do a show about undersea animals — all the ones that I’m interested in and know a lot about,” he told Entertainment Weekly in 2010. “I focused on the sponge because it’s one of the more peculiar creatures. …”

For the character’s voice, he called upon Tom Kenny, who had worked with him on “Rocko.”

“I told Tom I wanted the voice to be basically like a Munchkin from the ‘Wizard of Oz,'” Hillenburg recalled. “And he imitated, on the spot, this Christmas dwarf he had overheard in the mall. I said, ‘That’s it!'”

The first episode of “SpongeBob SquarePants” — pegged to a daffy, pineapple-dwelling fry cook — debuted on Nickelodeon on May 1, 1999. The series has aired nearly 250 episodes to date. Just last week, Nickelodeon debuted a new holiday special called “SpongeBob MerryPants.”

“SpongeBob” appealed not only to children but older viewers as well, including college students who organized viewing parties for the show. As a TV Guide writer pointed out: “Children adore SpongBob’s charming naiveté, teens and young adults love the filthy jokes for stoners hidden not so subtly in the background, and adults thrive on it’s meme culture which redefined the way we talk on social media.”

The toons of my teens and the memes of my twenties. Thanks for the giggles Spongebob. RIP Stephen Hillenburg pic.twitter.com/5xyj4PHDBz — Stephanie Walton (@stephaniewalt0n) November 27, 2018

Hillenburg also wrote, produced, and directed “The SpongeBob SquarePants Movie”, which was released in 2004 and went on to gross over $140 million worldwide. Hillenburg then wrote the story for and was the executive producer of the sequel, “The SpongeBob Movie: Sponge Out of Water,” in 2015.

In 2010, SpongeBob ranked 10th on Entertainment Weekly’s “100 greatest characters of the last 20 years.”

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

Fans and denizens of the show-biz world took to social media to pay their respects:

I first discovered SPONGEBOB through my sons, who were loudly singing along with the dopey & memorable theme song downstairs. I came down to see what the ruckus was about and ended up watching my first episode. Many laughs.

Many thanks, Steve Hillenburg. May you Rest In Peace. pic.twitter.com/3VNsxYJVXq — Brad Bird (@BradBirdA113) November 27, 2018

RIP Stephen Hillenburg. For my childhood and to this day 🤕🤦🏾‍♂️😭 pic.twitter.com/Nd1hK8fsxp — EG 💂🏾‍♂️ (@EG2Hunnidd) November 27, 2018

Stephen Hillenburg changed my life, and I didn't even know who he was….theres no logical reason that the hearing of his passing makes me so sad, but it does. Damn. pic.twitter.com/CIjqJqA10b — Constitution is Law (@TheTruth2See) November 27, 2018

So sad to hear of the passing of Steven Hillenburg, creator of @Nickelodeon’s @SpongeBob Squarepants. I had the joy of playing with his iconic creation in numerous concepts we developed for Paramount Parks. Go do something goofy and silly today in his honor. pic.twitter.com/meRv53v8Br — Dave Cobb 🏳️‍🌈 (@davecobb) November 27, 2018