We've all seen the movie poster for Sahara, and maybe even Raise the Titanic, but how about considering what the poster may look like when/if the other books are made into major motion pictures. Here's some samples created by talented graphic artist Andrew Huff.
Do you have any ideas?

another variation on Treasure:

The artist takes a little "artistic license":
"You'll notice I've included Al in it, even though I understand he is not present in the book. I figured Hollywood would rewrite him in to preserve the continunity of the films."

Variation of the same:

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