This tunnel is about a metre tall, and the same wide. There is no trace of the leaf matter that coated the previous areas, and the air smells old and stale. The fireball seems to have provided a very effective defence, and the dust on the floor is entirely undisturbed.
Shining the torch ahead, you see that the tunnel is curving around gently to the right, away from the theoretical position of the river outside. Unable to stand, but not so cramped you can't crawl comfortably, you advance forwards, following the curving passage.
Every minute or so the sound of the fireball comes from behind, and the tunnel lights up, but the sound is growing quieter as you leave the tubular area.