How do you get into 'Nightmare' mode? Enter the door to The Elder World. Jump in the pool, dive under the ledge you stood on. You'll land on a thick wooden beam. From here you can enter the slipgate.

There is a smashing 'Super Secret' stashed securely somewhere (Simon Says)! Enter 'Nightmare' mode - on the way to the slipgate, shoot the wall decoration, and Quake tell you "The Well of Wishes Awaits in the Crypt of Decay".

It does anyway - this doesn't trigger the secret. See 'The Crypt of Decay' how to get to it.