Randingo said:
What is an adventure wallet?
This wallet, dubbed a "hidden pocket" was designed by Karl Franz, author of The Peoples Guide to Mexico (first edition printed in early 70's and still reprinted/updated regularly).
Below, find a link to Karls' instructions for making one. His books illustrate the hidden pocket, but they are simple to make, even for a weary third world traveler in a seedy budget hostel with only the required fabric & a needle & thread (dental floss works well!).
I always wear one, or even two, when traveling. They can hold your passport, room key, travel documents, & quite large sums of cash inside your pant leg or skirt, & are guaranteed to foil pickpockets & even armed robbers. Best to keep your daily expense $ in your regular wallet. When secured by your belt loop they hang comfortably & invisible, under your pants/skirt.
The best fabric to make them is soft ripstop nylon--like parachute cloth. It can be bought in small amounts at just about any fabric store. Mine are black, which is practical. You will also find a few strips of soft velcro beneficial. Make sure to measure for size of a passport as well as your cash. While you are at it, it's good to make two. If you ever needed to abandon your rig, you'd want somewhere safe to secure your savings on your body.
Here is the link to Karls' Hidden Pocket:
http://peoplesguide.com/1pages/chapts/pack/pocket.html
Good luck!
Charlotte