An initiation ritual dating back to early maritime history, in which sailors are changed from scurvy pollywogs (or just 'wogs) into honorable and trusty Shellbacks upon sailing across the equator.
Those who cross the equator and international date line at the same time are initated as Golden Shellbacks. The initiation is similar for both.
The rites of the mystery are, of course, a closely guarded secret. What is known is that before crossing the equator, the wogs are run through a series of tests of their character, fortitude, ability not to vomit, and threshold for pain, by their trusty Shellback shipmates.
Upon crossing, Neptunus Rex and his Royal Scribe Davy Jones come aboard to charge the wogs of assorted crimes that they were certain they'd gotten away with. Those scurvy wogs who can prove their innocence and exemplify that they deserve to be Shellbacks after a rigorous inspection and interrogation are initiated by Neptunus Rex himself, and given a card and/or certificate which reads:
Know All Ye by These Presents:
that ___(name)____ on the ___ day of _____
aboard the ___(ship)___ appeared at the Equator
at longitude ______, thus entering Our Royal
Domain, and having been inspected by My
Royal Staff, was intiated into the Solemn Mysteries
of the Ancient Order of the Deep. I do hereby
command All Hands to Honor and Respect
this Trusty Shellback.
Signed Davy Jones _______________ Neptunus Rex_______________
His Royal Scribe_______________ Ruler of the Raging Main_______________