Where Scrooge McDuck
kept his "five billion quintiplitilion unptuplatillion multuplatillion impossibidillion fantasticatrillion dollars" (source: Carl Barks
). A huge, grey, cubic building (see further description below) which had an enormous "$" on it.
The money bin is three cubic acres big, most of which is devoted to storing cash -- Scrooge liked to "dive through it like a porpoise, burrow through it like a gopher, and toss it up and let it hit him on the head". (Barks again) However according to the story "The Second Richest Duck" we learn that the bottom of his bin is also filled with jewels and other assorted goodies ("a layer of star sapphires", etc. etc.)
Scrooge never had security guards on the bin, despite the riches contained within. Instead he placed it on the easily defensible Killmotor Hill, and surrounded it with various cunning traps (such as "Boer War surplus" cannons that would pop up and blast away at intruders). He also covered the bin with huge signs reading things like "Scram!" Effective security, because although the Beagle Boys and other villains often had wonderful schemes to break into the bin, they were rarely successful (and never permanently successful =).
Scrooge's main rival, Flintheart Glomgold, also had a money bin, which was nearly identical to McDuck's except that it featured a huge pound sign on the side! Indeed, Scrooge and Flintheart were shown to have nearly identical (or at least equivalent) riches in their respective bins (again from "The Second Richest Duck").