As the AD/HD video Scientifically Accurate Ducktales points out (WARNING: NSFW, also discussion of duck-on-duck rape and cannibalism), a giant bin of gold coins would be rock solid. Now, while I could just handwave and say that Scrooge swims in the money bin Because Reasons, it's much more fun and interesting to come up with some bullshit D&D reasons this works.
Basically, the money bin is magic. Super, super powerful ancient magic.
THE MONEY BIN (magical location)
In Jammer Hearts, as in the original TV show, Scrooge is a former adventurer. One of his last adventures before retirement involved delving into an ancient factory built by an unknown, long-extinct race. While in this factory, he discovered what would eventually be termed the Money Bin.
The Money Bin's power is that any metal placed inside of it below a certain threshold (indicated by a silver line inside, below a platform sticking out into the bin) turns into a room-temperature liquid. The liquid metal stays the same color as whatever metal it originally was, remains opaque, and is imiscible with other metals placed in the bin (so if you place gold and iron in the bin, you get a blob of gold liquid and a blob of iron liquid). In all other ways, the metal in the bin has properties identical to water: they gain the density of water (so lead doesn't sink to the bottom and tin doesn't float to the top), and become non-toxic (so you can swim in lead with no harm; best to make absolutely certain you wash it all off first!). However, unlike water, the liquid metal is also entirely inert and non-reactive, regardless of what properties it had before; you could put raw sodium in there if you wanted to contain it, or even radioactive metals.
Alloys separate into their component parts; for instance, if you put in steel, the carbon will separate from the iron and usually float to the top (non metal objects react as though the liquid had the density of water). This is a great way of purifying metals, and of getting metal out of ore.
Metals removed from the bin revert to their solid form instantly and regain all their normal properties. They take on the shape of whatever container is used to remove the metal. If you simply dip a cup into the top of the bin, you'll get a swirl of whatever metals happen to have been placed in there last, fused into a solid.
The Money Bin has a number of differently sized spigots at the bottom with a whole slew of levers next to it, all of which are labeled with the names of various metals. There are even a few blank ones, and slots for inserting more levers. Pulling an individual lever will place a filter on the spigot that will only allow that type of metal to be poured out. This can be a slow process, as if there's a glob of iron floating at the top of the bin, it could take some time for it to sink far enough to come out of the spigot (it will sink eventually, however). This allows for separating the metal slurry into pure metals. There's even a few levers for alloys, though the alloy components must be in the bin to allow pouring of alloys.
There's a second device, a giant siphon, at the top of the bin. The siphon can be moved around via levers at the top platform, and dropped into the slurry. The siphon can do the same thing as the spigot, but from the top, and it can drain larger quantities of metal (so if you were making a bigger object; the spigot is mostly for things you can hold in your hand, the siphon is for very large projects). The siphon also has another unique property; there's a chute through which you can dump worked objects. The siphon can remember the worked object in a database (accessible from the platform) and basically 3D print an exact copy of the object using any of the metals currently stored in it.
Magic items made of metal dropped into the Money Bin must make a Constitution save (or Save vs Magic Device, or Save vs Polymorph, depending on what edition you're using) every round it is in the bin or be destroyed. The magic in the object is undone and the object is melted down into its component metal. Obviously, there are some exceptions, and especially powerful or old magic items may be immune, or cause unpredictable interference with the Money Bin's magic (a keyblade, for instance, would likely not be affected).
The bin has one final ability: it can create a liquid metal guardian (or series of guardians!) to defend the bin. The guardian is composed of animate metal (this time with the same density as whatever metal it was created from -- often gold is used for this purpose simply for its great weight) and is not intelligent. Treat as an iron golem, only instead of a breath weapon it has an attack like a rust monster's. Instead of rusting items, it causes metal to turn to liquid and re-solidify into useless, twisted shapes.
Scrooge McDuck found the bin in his younger days, during one of his adventuring expeditions. The bin lay at the center of a huge, ancient, abandoned weapons factory, a place created by a forgotten race for a long-past war. The rest of his adventuring party were interested only in the bin's liquid metal contents, along with the various uncut gems in the bottom (from the earlier users throwing ore into the bin to separate the metal from the ore) and the weapons and items in the factory's store rooms. Scrooge, cannily, said that he wanted no part of the gold or items; his party could take the lot... in exchange for the bin itself. The rest of the party happily agreed. Scrooge then brokered a deal with an off-planet business for a one-year contract to provide exceptionally pure metal for use in manufacturing in exchange for the manufacturer moving the bin to his own home.
Within five years, Scrooge's adventuring partners had wasted all their gold on frivolous pursuits, and Scrooge was wealthier than all of them combined through the use of the Money Bin in manufacturing. Scrooge hired many top mages and scientists to study the bin's properties to see if they could replicate it, but to no avail as yet; the bin's magic is too ancient and strange. Scrooge recently halted all progress on researching the bin, though he did not cite any reasons why.
Now that Scrooge is the richest being in the galaxy, he doesn't use the bin for manufacturing as much anymore. He owns a number of subsidiary companies that provide him with more than enough money, and though he often grumbles about "parasites" and "people need to pull themselves up by their bootstraps" he secretly has a soft spot for up and coming adventurer types and secretly runs various charities to help young adventurers find their feet. Instead, he uses the bin for purely recreational purposes, as his own private, completely ridiculous, and very expensive lap pool.
> A bitter rival wants to destroy Scrooge's business. Said rival has no idea of the magic properties of the bin; he merely believes that the bin just stores Scrooge's gold. He hires the adventurers to steal from it.
> Someone has managed to secretly activate the Bin's Guardian, out of Scrooge's control. The monster is rampaging around Scrooge's estate, completely unstoppable. Scrooge blames David Xanatos, saying that Xanatos managed to steal the magical research done into the bin's properties, and was thus the only other person who knew about the Guardian. Xanatos denies that it was him, saying that his own estate was recently broken into. Scrooge doesn't believe Xanatos, and is working on legal action, even while he tries to hire people to deal with the rampaging Guardian. If it's not Xanatos, then who awakened the Guardian, and to what purpose?
> Something is stirring in the ancient factory that Scrooge retrieved the Bin from. What is it? Who built the bin in the first place? Does the factory really predate the last Heartless incursion, and, if so, what happened to the people who built it? Perhaps they have some secrets that will allow the universe to fight back... or perhaps they will only serve as a lesson in hubris.