I haven't seen this posted yet so I thought I'd do it. I checked through the topics list and in the secrets thread but if I missed it, sorry.
(http://i.imgur.com/248nP.png)
If I missed anything please let me know! Also, if anyone wants to label interesting areas/secrets/etc. on this map, feel free.
There's an additional passage near the top-right of the map (highlighted in yellow):
(http://i.imgur.com/d9cXSTX.png)
[mod]Added spoiler tags[/mod]
Edited with my known secret entrances
http://i.imgur.com/PgwXdgB.png