Apart from the games awesomeness, it does contain a lot of glitches and problems. This one however is rather beneficial.
First of all, you will need to equip the whirlwind sprint shout and go behind the Jorrvaskr, located in Whiterun. you should be able to see a sort of ledge, near the black smith that goes up and above the blacksmiths forge. Once you are here you will need to use the whirlwind sprint shout to get yourself onto the roof of the Jorrvaskr.
Once on the roof, you will need to position yourself looking at the small dragon monument on the roof, closest to the wall. I recommend you save here because it can be a little tricky!
Once you have saved, all you need to do is jump into the dragon pillar, until you are as high as possible, then you use the Whirlwind shout at the town wall and with a bit of luck you should get over it!
That is the hard part done, now all you need to do is find the hole in the map floor, which is somewhere in the rocky area next to the wall, once you stand on it, you should just drop down and under the map!
Once under Whiterun, you should be able to see a chest underneath Skyforge quite easily, you can open the chest and take what is in it! The value of the items in the chest increases with levels.
That is all! If you have any problems please comment below and I will get back to you. :-)