The First National Bank of SpongeBob is a black pot that belongs to SpongeBob. The only way to get there is through a rainbow. So far, it only appears in "Life of Crime." SpongeBob secretly keeps his money inside, unless he takes it all out or he runs out of money.
It looks like a black pot, similar to one aleprechaun has. A rainbow leads to it.
Role in episode
When SpongeBob and Patrick are looking for money to buy a balloon, SpongeBob looks in The First National Bank of SpongeBob but the pot is completely empty.
It is a reference to The First National Bank of America.