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

The cause of death was ALS, which Hillenburg revealed he had been diagnosed with in March of last year.

“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.”