Well, a tunnel horror story can revolve around a character being trapped in a tunnel during a disaster. For instance, a tunnel collapses and the person is stuck inside. They have limited air, and they start to hallucinate from the lack of oxygen and the stress. There are also the sounds of the settling rubble, which they mistake for something more sinister coming to get them.
A tunnel horror story could be about getting lost in a dark, damp tunnel. For example, a person might be exploring an old, abandoned tunnel system. They hear strange noises like echoing moans and the scurrying of unseen creatures. As they try to find their way out, their flashlight starts to flicker, leaving them in almost total darkness, surrounded by an oppressive sense of dread.