Sounds to me like this is something that should be doable with collision channels… But I’ve never gotten those to work the way I wanted. And you would have to recreate just about everything that happens with PHYS_Walking if you wanted to toggle collision channels on your player.
How about this instead?
Keep the entrance to the cave above ground, and put the underground section of the cave somewhere well below the landscape. Put in some trigger volumes to teleport the player from the landscape to the cave and back again. You could fake being able to see into and out of the cave with some camera actors and render textures. You could also mess with the post-process effects to hide any discontinuities, making the world too bright or too dark for a couple of seconds while the player’s eyes adjust to the light.