Stephen Hillenburg, the creator of Nickelodeon's long-running hit, SpongeBob SquarePants, has died at 57.

Hillenburg died of ALS, which he revealed he had been diagnosed with in March 2017.

"We are sad to share the news of the passing of Stephen Hillenburg, the creator of Spongebob Squarepants," Nickelodeon said in a statement. "Today we are observing a moment of silence to honor his life and work."

ALS, or amyotrophic lateral sclerosis, also known as Lou Gherig's disease, is a progressive neurodegenerative disease that attacks nerve cells in the brain and the spinal cord.that control the muscles. It is a rare illness, with no known cure.

Hillenburg, a former marine biology teacher, created the SpongeBob series, featuring an animated sponge that lives in Bikini Bottom in a pineapple under the sea, in 1999. The endearing animation, featuring a cast of goofy characters, started as a Saturday morning cartoon, then became a primetime multibillion-dollar franchise popular with kids and adults, spawning animated and live-action movies and even a stage musical.

Hillenburg also directed 2004’s The SpongeBob SquarePants Movie, and was the co-writer and executive producer on the 2015 sequel, The SpongeBob Movie: Sponge out of Water. He stepped down as a showrunner from the series for a decade before returning in 2014 as an executive producer.

USA TODAY contributed to this report.