LOS ANGELES (AP) — Stephen Hillenburg, who created SpongeBob SquarePants and the absurd undersea world he inhabited, has died at age 57, 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.

Hillenburg 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.

An Oklahoma native with a love of both drawing and marine biology, Hillenburg conceived, wrote, produced and directed the animated series that began in 1999 and went on to spawn hundreds of episodes, movies and a Broadway show.

“He was a beloved friend and long-time creative partner to everyone at Nickelodeon, and our hearts go out to his entire family,” Nickelodeon’s statement said. “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 absurdly jolly SpongeBob, his starfish sidekick Patrick, and a vast cast of oceanic creatures quickly appealed to college kids and parents as much as it did kids.

“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.”

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 an emphasis on marine resources, and went on to teach marine biology at the Orange County Marine Institute.

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

That same year he created an animated short called “Wormholes” that won festival plaudits and helped land him a job on the Nickelodeon show “Rocko’s Modern Life,” where he worked from 1993 to 1996 before he began to build SpongeBob’s undersea world of Bikini Bottom, 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, “and that there is no organized community with roads, where cars are really boats. 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. Its nearly 250 episodes have won four Emmys and 15 Kids Choice Awards, and led to an endless line of merchandise to rival any other pop cultural phenomenon of the 2000s.

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.”

A musical stage adaptation bowed 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.