You lost me here...
I`ll try to explain, but if I dont manage to, I`ll draw you a picture in paint
Anyway... have you ever seen things like this in movies? A waterfall falls down from a mountain, and when some Indiana Jones character goes through the waterfall, the mountain turns out to be one hoge chamber... empty inside... hollow.
So what I had in mind here, Mind

, is for you to make this mountain one big wakable doodad, with the water fall at its one side. Now you could probably pull out a trick with triggers, that would allow you to remove the mountain, or just hide it and make its wakable pathing disapper, when you go into the waterfall. So basiclly it would look like you entered the mountain when you go throgh the water... and ofcourse you would have to bring back the mountain when a unit leaves the chamber.
Now about the inside... you would have to spawn the items and the creeps inside the minute a unit enters, so they wouldnt turn out to be on top of the wakable mountain when you are not in the chamber, and hide or remove them when a unit leaves.
It is a hard task, but it would create a beautifull effect. Good luck.