Letterboxing?

Has anyone heard of this at all? This sounds kind of interesting. When I go walking around I like to explore a little, and day my day I'm kind of running out of areas near me that are new. This idea seems interesting. People leave a waterproof box with a rubberstamp and a paper pad inside somewhere in town and hide it. Then they put clues online like such. These are all the Oregon ones listed by date placed. Some were placed in 1999, so I'm sure those are gone. Anyway, it seems like a cool alternative to doing nothing.