However, a strong contender for the title is the true nature of Spongebob's immortality.
Here's why this secret is significant:
* It's hinted at, not explicitly stated: Throughout the show, Spongebob exhibits unusual resilience and seemingly impossible feats. He survives being crushed, burned, and even flung into space with minimal injury. While this is often played for laughs, it raises questions about his true nature.
* It has implications for the world: Spongebob's immortality could explain why Bikini Bottom seems stuck in a perpetual state of normalcy. Even with the bizarre events that occur, things always return to the status quo.
* It could be a plot point in future episodes: The mystery of Spongebob's immortality could be explored further in future episodes, potentially leading to some interesting twists and turns.
Ultimately, the "biggest" secret is up to individual interpretation. However, the nature of Spongebob's seemingly supernatural resilience is a compelling mystery that could hold the key to understanding the world of Bikini Bottom.