Hello Hoteloscar! I might be able to help you with your question, unless you've found it already. Of course, my answers contain SPOILERS, but this should be of help to anyone at the end of their rope as well.

Go to the Hall of Magic and stand facing the enchanting table in the corner. Turn so you are facing the garden. Drop into the pool at this corner and look to your right. There is a boarded over entrance (not unlike the entrance to the Thieves Guild) that will take you into the Underdepths. BE WARNED - have an item enchanted with a waterbreathing spell! There is some swimming through a maze involved. Also make sure you have a modest supply of lockpicks handy.