well to be serious, probably a pyramid 100x100 blocks from the bottom going up, all made of wood planks and 100% filled with wood planks too. it took me about 3 days. and it was for the xbox like, almost a year ago.
i went to a corner and llet a 4x4 of water i dag deep and to the right the triki part was that i had to keep comingback and i made like a batman cave because u could only ente trhough the bottom... it was hard BUT WORTH EVERY SECOND :)