The letters you merge into a mysterious island (you need 4 letters) and you get coin bundles and gems from that. and also every once in a while itll spit out a mysterious fossil. Have Fun!!

The envelopes come from the shell bucket and if you click the bucket super fast to get a lot of pebbles, it will randomly spit out a mysterious letter. I am on level 15 and i have gotten 4 mysterious letters from doing this so now I have a mysterious island. The mysterious island usually popes out coin bundles and gems, but every once in a while it will pop out a mysterious fossil. Have fun!!