The surface was near. Stella could feel it, she knew how deep in the ground she still was. Unless Twilight had raised up a mountain on top of the tunnels, which she doubted was the case. The time for it hadn't been there. Neither did it seem like the kind of thing the mare would want to do. She was most likely content with extending their confrontation within the tunnels, but not too much further. And if she did, Stella wouldn't allow it. Not that it would be hard to.
Most of the tunnels were in Stella's control at that point. Twilight was hiding in the upper portions, and likely retreating quick. Stella's control over the spell was growing fast, spreading upwards through the tunnels, slithering along their walls and overtaking and twisting Twilight's spellwork. She'd actually have to thank the other alicorn for something, it was a valuable learning experience and something she'd certainly enjoy replicating. She could even expand the tunnels and turn them into another hideout if she felt like it in the future, or build her own system. The place was appropriate for it, far enough from everything else as Twilight had said.
Maybe she really would do that. Maybe not. Maybe the whole area would end up destroyed during the following battle. Stella really did have to thank Twilight. The slow walking and methodical exploration had really helped with clearing out her head, and though the wound on her neck still hurt she was no longer as unstable as she'd been a while before. She was feeling a lot better, a lot more rested. If Twilight had hoped to tire her out by trapping her there she'd made a huge mistake.
There was also of course another possibility. Twilight may be trying to buy herself time in the hopes help of some other kind would somehow arrive, maybe trying to contact someone in some way. It wouldn't work of course. Stella suspected if there was anyone Twilight might try to get to, it was either Sunset or her more recent and more unfortunate acquaintance. The latter was a purely desperate move and it would do her no good. The stallion was a nuisance in his ability to see through Stella's coil, but dead weight on the battlefield without time to set up any traps. The former option simply wasn't there, Sunset had returned to her world and Stella had made sure to prevent her from coming back for a day or so.
But even if either or even both of them got there, what would it accomplish? She could still force Twilight to fight by herself, she had her friends hostages. Some willingly so. Celestia was most likely capable of breaking out of her imprisonment, but was obviously aware that doing so would doom someone else. She'd chosen to be captured herself after all. She appeared to have an unjustified faith in Twilight. No matter. She'd remember what Stella decided when everything was done with.