I think you are not know that it exists.
The GameDesigner Josh Foreman have drop many mysterious coins. I think 26.
This coins are on secret area and josh have drop coins to say to the guys :
“You might find yourself in some out-of-the-way places while you’re crab hunting, so keep an eye out for Josh’s signature coins. He plants these in areas he refers to as “negative possibility space”—places that players can reach with effort, but that contain no content. The coin is an acknowledgement that explorers have done something cool by finding a way into those places.” ~ Joshua Foreman (Game designer)
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We can found :
1] The news :
https://www.guildwars2.com/en/news/points-of-interest-livestream-summary-lions-arch/
2] The wiki page :
http://wiki.guildwars2.com/wiki/Mysterious_coin
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YOU KNOW WHAT TO DO NOW ! GO HUNT !
