I've seen several res with these donation pits. There is usually a sign asking you to throw unwanted item into them (nearly broken tools, spare cobble, dirt). They usually consist of a hole with flowing water going out of sight. At first I thought these would drop the items down to bedrock layer where there would be no players within range to cause them to despawn. I now know an item despawns after being 5 mins within a loaded chunk and all the chunks on a res of a popular shop would be loaded quite often.Do these donations just despawn and are a futile gesture or do these donation pits have some way of keeping the items spawned?