In the Black Hand's Grasp level, cathedral, hedge maze, etc.

I've got the red key and have killed all enemies on the board. However, I cannot find the passageway/door/whatever that allows me to go down into the 'bowels' and retrieve the scientists.

Where is the door/passageway/entrance/whatever?

Thanks.

PS. Renegade is an absolute hoot. If there isn't a sequel in development then everyone at westwood should be arrested !