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Stephen Hillenburg, who used his dual loves of drawing and marine biology to spawn the absurd undersea world of SpongeBob SquarePants, has died, Nickelodeon announced Tuesday.

Hillenburg died Monday of Lou Gehrig’s disease, also known as ALS, the cable network said in a statement. He was 57.

He had announced he had the disease in March 2017. His death comes just weeks after the passing of another cartoon hero in Marvel creator Stan Lee.

Hillenburg conceived, wrote, produced and directed the animated series that began in 1999 and bloomed into hundreds of episodes, movies and a Broadway show.

The eternally, obliviously jolly SpongeBob and his yell-along theme song that opened “Who lives in a pineapple under the sea?!” quickly appealed to college kids and parents as much as it did kids.

“The essence of the show is that SpongeBob is an innocent in a world of jaded characters,” Hillenburg told The Associated Press in 2001. “The rest is absurd packaging.”

Its vast cast of oceanic creatures included SpongeBob’s starfish sidekick Patrick, his tightwad boss Mr. Krabs, and his always-exasperated neighbor Squidward Tentacles. Hillenburg’s undersea world of Bikini Bottom was a realm like no other, real or fictional. SpongeBob can play his nose like a flute and could not possibly be happier to work his fast-seafood job of flipping Krabby Patties.

But he has his troubles, too. He constantly fails his boat-driving test, forcing his frightened blowfish teacher to inflate. In one episode he suffers a broken butt and is afraid to leave his pineapple home for days.

“I don’t want to face my fears,” SpongeBob, voiced by Tom Kenny, says in another episode. “I’m afraid of them!”

Kenny presented Hillenburg with a special honor at the Daytime Emmy Awards in April, and said in his speech that Hillenburg himself is the embodiment of his main character.

“SpongeBob’s vocal cords might be mine,” Kenny said. “But SpongeBob’s playful spirit of gentle anarchy, his humor and the joy he takes in his vibrant, colorful, music-filled world come directly, directly, 100 percent from my good buddy, Mr. Stephen Hillenburg.”

Born at his father’s army post in Lawton, Oklahoma, Hillenburg graduated from Humboldt State University in California in 1984 with a degree in natural resource planning with a marine emphasis, and went on to teach marine biology at the Orange County Marine Institute.

While there he drew a comic, 'The Intertidal Zone,' that he used as a teaching tool. It featured anthropomorphic ocean creatures that were precursors to the characters on “SpongeBob.”

Hillenburg shifted to drawing and earned a master of fine arts degree in animation from the California Institute of the Arts in 1992.

He worked on the Nickelodeon show Rocko’s Modern Life, from 1993 to 1996 before he began to build SpongeBob’s universe, which showed off his knowledge of marine life and willingness to throw all the details out the window.

“We know that fish don’t walk,” he told the AP in 2001. “And if you know much about sponges, you know that living sponges aren’t square.”

The show was an immediate hit that has lost no momentum in the nearly 20 years since its creation and helped define the culture of Nickelodeon.

“He was a beloved friend and long-time creative partner to everyone at Nickelodeon,” the network’s statement said. “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.”

Its nearly 250 episodes have won four Emmy Awards and led to an endless line of merchandise to rival any other pop cultural phenomenon of the 2000s.

“When you set out to do a show about a sponge, you can’t anticipate this kind of craze,” Hillenburg told the AP in 2002.

In 2004, the show shifted to the big screen with The SpongeBob SquarePants Movie and a 2015 sequel, The SpongeBob Movie: Sponge Out of Water. David Bowie, Johnny Depp and Scarlett Johansson are among the dozens of major stars who provided guest voices to the show or movies.

Intensely involved in every aspect of the show initially, Hillenburg after the 2004 film stepped back into an executive producer role on the show, where he remained for the rest of his life.

A musical stage adaptation debuted on Broadway in 2017, with music from such stars as Steven Tyler, Sara Bareilles and John Legend. It earned 12 Tony Award nominations, including one for best performance by a leading actor for Ethan Slater.

“I am heartbroken to hear of the passing of Stephen Hillenburg,” Slater said in an email Tuesday. “Through working on ‘SpongeBob,’ I got to know him not only as a creative genius, but as a truly generous and kind person. He warmly embraced us on Broadway as the newest members of his wonderful ‘SpongeBob’ family, and made it so clear from the get-go why he is so beloved: genuine kindness.”

Hillenburg is survived by his wife of 20 years Karen Hillenburg, son Clay, mother Nancy Hillenburg, and a brother, Brian Kelly Hillenburg.