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The Big One is a massive tidal wave with a wicked smile and the titular antagonist in the SpongeBob Squarepants special SpongeBob Squarepants vs. the Big One.
History
JKL explains to Spongebob, Patrick, and Squidward that The Big One can be strong enough to take the three back home. But, one will have to sacrifice hiself/herself for the rest to make it back alive. The three, as well as Mr. Krabs and his cash register Cashy, attempt riding it, but to no avail. Both Patrick and Squidward's boards get "eaten" by The Big one's mouth. When JKL arrives and soothe's the wave's rage, the four (all on Spongebob's board) celebrate until Mr. Krabs drops Cashy into The Big One's mouth, angering it again. JKL sacrifices his life and jumps in after Cashy. Spongebob plays JKL's drums, and The Big One takes them home.Spongebob, Patrick, Squidward, and Mr. Krabs all make it back to Bikini Bottom safe and sound. JKL is revealed to be alive for unknown reasons, as well as Cashy, ending the episode.
Nature
Appearance
The Big One is a large, blue wave of water with a face and tongue made of water. Once, it smiled and ate Patrick's surfboard.
Emergence
In every year ending in -009 (e.g. AD 1009) during the alignment of the planets in the solar system, The Big One appears. The Big One will next appear in 3009, 1000 years after the previous one (2009).
Behavior and Sentience
The Big One's sentience was shown when it attack any surfers whom used it's body as advantage to escape the island in it's territory. In the episode's case, the target was SpongeBob, Squidward, Patrick, and Mr. Krabs (Cashy also included). JKL explained that the motivation of the attack was it demanded a sacrifice from those whom used it's body as the payment of using it. Aside sacrifice, it can be appeased with playing the drum with the same model of JKL's, which also allowed the Big One to bring them straight to the beach near the player's home.
In spite of it's demand of the sacrifice, it can be merciful to the said sacrifice and spare him/her. But the only example was JKL, and the reasons was unclear, if his heroic sacrifice doesn't count.
Mystery
As it only appeared as one-shot antagonist, many more fact about it yet to be revealed because it can only emerges every 1000 years (see Emergence section above). Because of it demanded the sacrifice and the daily occurence was once per 1000 years, Big One could be a god of the island where it appeared or an elemental being, or anything, which the answer was yet to revealed until now.