Tunnels of Terror - Level 4?

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The "Tunnels of Terror - Level 3D, Section 2" has a passage leading to Level 4. I have been unable to locate that level anywhere. Is it a passage back to Rappan Athuk Level 4 or something left for the GM to expand upon?

Thanks!

Drcritique's picture

The ToT are connected to RA but RA is not connected to ToT. 

Hope you can understand my thinking.