My brother and I put a corked glass bottle down in an old defunct drainage pipe beneath my parents' house. This pipe/canal is quite large and isn't obstructed by the bottle, and the bottle can clearly be seen by peering into a hole in the cement of the basement storage room. Inside of that bottle is a carefully folder paper bearing on it a crude drawing of a cock and balls.
It was likely a permanent Sharpee marker. Hopefully it holds up. Fingers crossed that I'm able to return there as a ghost one day to watch someone unearth what they believed was a map to the family treasure.