'Lumos' you mutter. You sort of regret it as it illuminates what you're standing on. Bones. Lots of tiny bones, most likely rats. The air is so thick with moisture you can feel beads of water collecting on your skin. You take a quick look around, the green mossy brick walls surround you, the tunnel goes straight for a couple of meters but then forks out. You keep your wand and mind at the ready, preparing a couple of spells that you can use if the need arrises.
You need to hurry and find Ginny before it's too late.
"It was like rushing down an endless, slimy, dark slide. He could see more pipes branching off in all directions, but none as large as theirs, which twisted and turned, sloping steeply downwards, and he knew that he was falling deeper below the school than even the dungeons. <...>
and then, just as he began to worry about what would happen when he hit the ground, the pipe levelled out, and he shot out of the end with a wet thud, landing on the damp floor of a dark stone tunnel, large enough to stand in. "