Welcome to the forums!
I think the easiest way to make people feel underground would be to build a platform maybe 1-2 feet off the ground going back to a few scenes before the descent. You could certainly make use of the ramp up for theming too, but once guests are up, the goal would be to give them enough time to forget that they had gone uphill. Then, you could send them down the ramp into a cellar/sewer themed set and I think it would feel a lot more realistic that way.
Of course, if you couldn't do the ramp up, there's a few other ideas you could use to simulate the feeling of being underground:
-Ceilings that get lower going down a hallway
-Building a fake staircase into the first "underground" scene to establish the setting
-Just theming the scene as a cellar will get you 90% if none of the above are plausible. (Keep in mind that guests are more focused on what's up ahead versus the believeability of how they got to a certain setting.)
Hope this helps!