The cave is dark. Bella has a glowy thing, which she takes out and hangs around her neck; the cave doesn't seem inclined to unpleasantly surprise them by preventing the glowy thing from working.

The cave narrows to a tunnel just barely wide enough for them to proceed single file.

Bella follows Sherlock in for twelve steps, and then is swallowed up by the floor with a yelp. The floor closes after her. It doesn't dampen the sound of her in her armor clattering to uncharitably distant ground, or her whimpering the words of her healing spells.