On Monday, SpongeBob SquarePants creator Stephen Hillenburg died after a recent diagnosis with ALS. Nickelodeon confirmed the news on Twitter Tuesday afternoon. What followed was an outpouring of grief for the man behind one of the most recognizable and beloved cartoon characters of all time.

In the 19 years SpongeBob SquarePants has been on the air, it’s become ubiquitous online and in real life, earning over $13 billion in merchandise sales, according to the New York Times. You can’t go near a school without seeing SpongeBob backpacks, you can't go through a Halloween without costumes of the cast parading past your door, you can't go online without being blitzed by SpongeBob memes and reaction GIFs. SpongeBob’s irrational, indefatigable optimism has become a big-eyed beacon of hope to anyone having a bad day, week, or 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.”

Though he created one of the most popular franchises of all time, the marine biologist-turned-animation icon was something of a mystery. Hillenburg rarely gave interviews, to the point that the New York Times once called him “animation's Howard Hughes.” He announced his diagnosis with ALS, or amyotrophic lateral sclerosis, in a statement to Variey in 2017. He spent most of his time with his family, surfing, or working on SpongeBob.

Here's what is known about him and his show: SpongeBob the character can be traced back to Hillenburg’s days teaching at the Orange County Marine Society, where he designed a comic book called The Intertidal Zone to introduce kids to different sea creatures. One of the main characters was called Bob the Sponge, an early ancestor of the square-panted invertebrate who lives in a pineapple under the sea. Hillenburg quit his job to pursue art in 1987, and in 1992 he graduated with an MFA from CalArts’ prestigious experimental animation program (alums include Coraline and The Nightmare Before Christmas director Henry Sellick, Kung Fu Panda director Mark Osbourne, Disney legend Glen Keane, and Rugrats producer Paul Demeyer).

He got his start as a director for Rocko’s Modern Life, one of the OG Nicktoons and ground zero for stupid-smart animated shows designed to please both kids and adults. That’s where he met Tom Kenny, the prolific voice of Heffer Wolfe, SpongeBob, and later Ice King on one of SpongeBob’s successors, Adventure Time. A writer for Rocko’s Modern Life encouraged Hillenburg to turn The Intertidal Zone into a series, which eventually grew into the adventures of SpongeBob, Patrick Star, Squidward, Mr. Krabs, Sandy, Gary the Snail, and the rest.

Through his show, Hilleburg was an evangelist of sorts for the unstoppable power of positive thinking, which he usually dramatized with absurd scenarios. Think of the time SpongeBob sculpts a perfect marble sculpture with a crack of the chisel, or when he wins a fast foodery face-off against the Flying Dutchman—the undead daddy of burger grilling—with the special ingredient of love. SpongeBob tackles everything in life—work, driving school, friendship, pain, lifeguarding, climate change—with a level of zealous breeziness usually reserved zen monks and six-year-old kids.

"The show is about watching an innocent character in this world that he lives in,” Hillenburg told the New York Times while promoting the first SpongeBob film in 2004. “It's saying that the childlike mind is OK. It's saying that dorks can be really important."

His values often spilled into real life, like when he openly dissented with Nickelodeon’s decision to do merchandise collaborations with fast food companies. “In the show, the whole point of the fast food—the fact that SpongeBob loves being part of the fast-food chain, and that being a manager is his ultimate dream: it's ironic,” he told the Times. “We didn't want to suddenly become the people serving up food that's not that good for you—especially kids.”

Hillenburg’s idealism led him to step down as showrunner after the movie. He reportedly originally wanted to end SpongeBob after the third season so that, as storyboard director Sam Henderson put it to Cartoonician, “the show wouldn’t jump the shark.” However, he stayed on as executive producer, reviewing each episode before it aired, and he returned to the show full-time in 2014.

The show's influence can be seen in the current generation of woke, emotion-driven cartoons like Adventure Time, Steven Universe, and Bojack Horseman. Writer Nick Jennings linked up with Hillenburg on the Rocko’s Modern Life team, then went on to help him create SpongeBob SquarePants before eventually becoming the adult in the room on Adventure Time when Cartoon Network still saw it as a risky gamble. Kent Osborne helped Hillenburg to write the The SpongeBob SquarePants Movie before moving on to the the show, and then helping write on Adventure Time. Now he’s writing on one of its successors, Julia Pott’s Summer Camp Island, a spiritual grandchild of SpongeBob.

More importantly, SpongeBob cemented zany animated television’s place in the mainstream. It outlasted predecessors like Rocko’s Modern Life, contemporary Nicktoons like Doug and Ahh! Real Monsters, and even successors like Adventure Time. (There's a third movie slated for 2020; the show is currently in its 12th season and it's unclear if it'll come back for a 13th.)

Even if you haven't seen an episode, you've likely been exposed to SpongeBob's many, many memes. Every rapper from Lil Yachty to Lil Pump has embraced the sponge. Twitter and Reddit regularly mine episodes—particularly from its golden first three seasons—for highly relatable moments, yielding gems like Evil Patrick Star and Tired SpongeBob. Mr. SquarePants even spent a stint as a Che Guevara-like symbol of revolution for Egyptians protesting Mohamed Morsi. Brand consultant Greg Rowland once compared him to Jesus. His creator is gone, but SpongeBob is going to live in that pineapple for a long, long time to come.

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