Hi, @wpnfcj, I assume from your post that the terrain is only a rendering glitch and disappears once you come closer and the tunnel renders correctly. Does increasing your graphics solve your problem?
You can try unticking the StreamingEnabled property in the Workspace, sacrificing performance to solve the rendering glitch.
This is a widespread issue with Roblox terrain and currently has no proper solutions to resolve it.
Consider setting the StreamingTargetRadius to a high number and StreamOutBehavior to Oppotunistic, this should reduce the rendering problem from being seen. This may impact performance.
Or
If your tunnel is deep-within the mountain, then I would recommend not adding any terrain surrounding it, which can both improve performance with less terrain and resolve the issue.
Yes, leave the interior of the mountain empty if it isn’t necessary, extra chunks of terrain can impact performance and is completely unseen by the player.
It’s caused by some bug in the client possibly? I have the same issue on games on different devices so it might just be how low your studio rendering is.