When a ship approaches the Equator, crew members who are crossing for the first time must appear before King Neptune & his court to demonstrate their worthiness as subjects of the sea.

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Crossing the Line: Violence, Play, and Drama in Naval Equator Traditions. -

Proof is exacted through tests & punishments that can range from the mildly embarrassing—singing a song or reciting a nonsensical rhyme—to much more grueling treatments: running the gauntlet, tarring & feathering, or crawling through slops. The custom earns the sailor or passenger little more than a certificate & the right to call himself or herself a shellback (click to view example of certificate)