Stephen Hillenburg, the creator of SpongeBob SquarePants, has passed away, Deadline is reporting. Hillenburg had been battling an ALS diagnosis for two years, and died on Monday at the age of 57.

Though he began his professional life as a marine biologist educator, Hillenburg would become far better known for his nonsensical, decidedly non-educational cartoon series. Premiering in 1999, SpongeBob SquarePants quickly became Nickelodeon’s most popular show ever, delighting not only children, but plenty of adults as well (according to estimates at the time, roughly one third of SpongeBob’s audience was over the age of 18). SpongeBob won critical acclaim for its splashy animation and irrelevant humor, and a theatrical feature–The SpongeBob SquarePants Movie–made the character even more popular when it opened in 2004.

Hillenburg left the series from a creative standpoint after the movie’s completion, which some fans claim lead to a drop of quality in the show as the years went one. Nevertheless, SpongeBob only became bigger and bigger as time went by, and the second theatrical film–Sponge Out of Water–made even more money than the first one had. SpongeBob has also recently made the successful leap from screen to stage, with SpongeBob SquarePants: The Broadway Musical becoming an instant hit in 2017. A third SpongeBob film is currently in production.