We have seen some interesting posts along this line of thought that makes a guy consider just how much Gold/Silver jewelry and coins are in waste pipes...
Cupajo once got a call to retrieve a gold ring that got flushed...He actually recovered it...Good Story...it made it all the way to the septic tank...Another poster takes his Garrett Carrot along when he is on business trips and staying in Hotels...he quickly checks the undersink plastic trap for earrings, small chains, etc that get accidentally washed down the drain in the sink, and caught in the 'P'trap...easy retrieve, no tools required to unscrew an undersink trap and dump out the goods...
I myself once made a tool out of a bent spoon attached to a long supple willow branch, fished it into the basement washing machine old iron pipe drains, and raked out enough money to buy a loaf of bread and a gallon of milk..
So yeah...lots of Gold and Silver down in the sewer waste pipes, hung up in the joints or traps......if a guy could figure out a way to get at it easy with a little robot retriever or something, it could be a viable concept....Lots of Gold rings get flushed down the toilet in a fit of rage, and if they get hung up in some paper swaddling, could travel a ways, not all the way to the waste water plant, but they will surely settle out somewhere along the way though......Theres about no way they will ever be found unless accidentally during an Infrastructure upgrade.. ...
Someday, somebody will make a massive gold haul out of a specific under ground sewer sump or trap that caught everything out of an old neighborhood...probably 50 rings settled into one little pocket, pile of old coins, gold teeth, imagine....