Get into the mine. If you are quick on your feet, you'll get to this corner. Go a little back up, you can get on top of the pipes. The actual secret is in the corner.

After you have smashed the control room completely, you can jump on a twisted bit of rail, and up on the rocks. There is a light embedded in the topmost rock (i had an electrician like that working for me once) - when you step on it, a secret door opens in the remains of the control panel.

Push the switch here - it's inside the blue door. Jump on the conductors as they retract, and all the way up on top of the cable tray. Follow the cables to reach the secret.

Now, eat a handful of your favourite brand of headache pills, and make your way out.