Here's a virtuoso piano performance of the Super Mario Bros. theme - using one hand. If I'd remembered anything from my piano lessons beyond playing Old MacDonald on the black keys, I could probably describe how difficult this is.
A commenter asks "Was that one hand for left AND right hand, or was it just a really intense right hand part?" and the artist, RigorMortis999 replies, "the former becoming the latter."
Good thing Mario didn't run down to his final 100 seconds at the end.
Mario Theme with One Hand Only [YouTube User RigorMortis999, thanks Isiah O.!]