In Blood Omen you can acquire a mist form spell that lets you cross water, and IIRC go through metal gates. I used it to walk into a big lake, not expecting anything, and what do you know, there's a YUGE secret area, an island with a teleporter to a secret werewofl temple.
I also really liked Avernum 1 for having so many damn secret ways in it.
I also really liked Avernum 1 for having so many damn secret ways in it.