If the amount of water actually caused the ground itself which contains the coins to become a liquid,then it’s possible for the resulting “soup” to no longer be able to hold the coins,causing them to sink to a more stable soil matrix. Firstly,this is pretty unlikely. Secondly,the result is the same...they’re buried under more material! According to what people have seen over the years,coins tend to get buried by the ever changing environment more than they just “sink” by themselves because of their weight. Even very light and loamy soil will hold a dime for a long time,it’s usually animal or human interference in the environment which changes their position appreciably. Hardwood trees which produce a lot of leaves and nuts can deposit ALOT of material onto the ground in a relatively short time,say 20 years. A coin starting on the surface and winding up 8” underground wouldn’t be uncommon in this scenario. But if you set a dime in a vase of sand and put it on the shelf in your house,it won’t go anywhere to speak of in that same time. There’s no reason for it to. Just my take on it....