Stephen Hillenburg was a marine biologist when he decided to go back to school and study experimental animation. The gamble worked, and Hillenburg’s legendary creation, the Nickelodeon TV show “SpongeBob SquarePants,” combined his two loves as an animated series about the lives of creatures that live under the sea.

On Tuesday, Nickelodeon announced that Hillenburg died Monday at the age of 57, after suffering from ALS.

“We are incredibly saddened by the news that Steve Hillenburg has passed away following a battle with ALS. He was a beloved friend and long-time creative partner to everyone at Nickelodeon, and our hearts go out to his entire family,” the network said in a statement. “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 animator announced a little over a year ago that he had been diagnosed with ALS, or amyotrophic lateral sclerosis. Although the disease progresses at varied rates, ALS is ultimately terminal.

Hillenburg created “SpongeBob SquarePants” after working on Nickelodeon’s “Rocko’s Modern Life.” “SpongeBob” became a massive success, much to Hillenburg’s surprise.

“I figured we might get a season and a cult following and that might be it,” Hillenburg told The Washington Post in 2009, speaking at the 10th anniversary of the “SpongeBob" television run.

Instead, the cartoon became a mainstay for a generation of children and evolved into a persistent cultural touchstone. SpongeBob starred in a 2004 movie written, produced and directed by Hillenburg. The television series has been dubbed or subtitled into more than 60 languages. Recently, “SpongeBob” became a Broadway musical, nominated for 12 Tony Awards.

Fans and fellow animators reacted to news of Hillenburg’s death with stories about the enduring cultural relevance of “SpongeBob SquarePants.”

“This was a man who put joy and love in the world,” tweeted Chris Nee, the creator of the animated children’s series “Doc McStuffins.”

“A giant of cartoons has left us. A kind, brilliant and hilarious genius who will forever be remembered for his creations,” wrote Jorge R. Gutierrez, an animator and director of the 2014 film “The Book of Life.”

Science-fiction writer John Scalzi tweeted that “SpongeBob and his Bikini Bottom pals are in the common culture now in ways only a few things ever manage.”

Many fans referenced the cultural relevance of the show’s catchy theme song.

“Once in grad school, a friend and I got the SpongeBob song stuck in our heads,” author Celeste Ng tweeted. “My friend got out of the car still singing ‘Who lives in a pineapple under the sea?' and a passing toddler on a tricycle was so excited he yelled out ‘SPONGE BOB SQUARE PANTS!’ and fell off."