Thanks for the compliment
There are a number of shipwrecks with various coins and bullion that have been discovered over the years. In this case, I decide to see which one I could find that had been the deepest for the Marianas Collection line. I discovered that there was a ship (the SS Gairsoppa) that sank in 15,500ft of water carrying silver bullion during WWII. The silver was recovered and can be purchased at various places these days as collector bullion. So I purchased some 1oz pieces and alloyed it into Sterling silver for the collection.
For my 18k work, I sourced silver and copper from the Atocha (a treasure galleon that sank of Key West during a hurricane in 1622) and alloyed that with 24k to get my 18k Treasure Gold.