I've dived down to the area east of the main building, and pulled the
lever that loads the dungeon. However, after swimming through a long
corridor inside, I get to a door with the picture of an octopus that
I can't open. There is no keyhole, and I cannot target the door with
the cursor. I've cast Spirit Eye, but there's nothing on the automap.
What must I do to open this door?

Regards,
Lars

Hmmmm I suggest you exit and look for another entrance. I think you happened onto the one that you have to open from inside. There are at least 2 more entrances that I know of and believe they are probably NE of your location (if you are near the 2 chests?)

Wyv

The obvious entrance near the two chests is actually the exit. The true entrance is hidden under one corner of the temple. The third area has a square hole leading down into a room. Does this actually go anywhere? I couldn't find anything in it or near it. I finished Collasium without ever using this area. Maybe it's just decoration, can anyone help?