Double Maps

Just how many good maps were ruined on suspicion of being a double map...

Most maps are fairly simple - a piece of paper or parchment with some ink on them. Roads, towns and all that. Well, most people cannot read even normal maps - but that is beyond the point. At first glance, double maps are just like all other maps. The second glance is what these kind of maps is all about, revealing a completely different map should you know the right approach.
The simplest way would be...to submerge it in water. Now now, I know that sounds like an insanely stupid idea! Could not have believed it myself at first. Turns out, the double maps have a hidden layer underneath the normal ink, made with some special water-resistant ink. Best not to leave it underwater for too long though, the actual map material is usually not as water-resistant as the ink is. You would not want a perfectly readable map to just fall apart at touch, now would you?

Mechanics & Inner Workings

Paper and ink. No seriously, what did you expect? It is a map, for God's sake. Granted, the ink is very special. And the "non-painted" parts of the top layer is also a special (not very special, mind you, just special) ink that imitates the colour and texture of the map's material. But at the end, this is all a double map is. With skill and time, it is even possible to carefully remove small parts of the map's ink to check whether there is anything underneath.

Manufacturing process

Wish I knew. Would have lived a life of wealth and comfort instead of writing guidebooks. Ahem, where was I. Nobody quite knows how these maps are made, or where, or by whom. I heard that it is an ongoing bet among the alchemist circles, who will manage to recreate the ink's formula first. Honestly, you would think making a waterproof ink should not be as big of an issue as it seems.

Rarity

Pretty rare, which is the problem. Records known of about three dozen double maps, but most probably never got reported. Besides, how do you prove that the map really was a double map once its upper layer has been removed? Still, if you see someone washing a map because - and I quote - "it looks too good and bad at the same time, I can tell this is a treasure map!", most likely you are looking at an idiot destroying a perfectly normal if somewhat disheveled map.