What is a shellback certificate?

Shellback certificates are awarded to Marines and Sailors who cross the equator while underway. Its a very old tradition. I couldnt tell you if its US or UK without looking. In the US it is celebrated by spending the day playfully hazing the new shell backs. People who have crossed before will spray new crossers with hoses and set up little challenges for them to run. It would only be worth a lot of money if it was awarded to a famous Sailor or somehow linked to something other famous. Other than that it is just something cool to have.