I remember once studying this story in high school and being given the assignment of having to draw a map of the tunnel! You might actually find this a helpful exercise to do in order to familiarise yourself with the tunnel and its various dangers. We know that there is a rockface in the middle of which the tunnel begins. When Jerry finally enters the tunnel, he finds himself "in a small rock-bound hole filled with yellowish-grey water." The roof of this tunnel is sharp. The width is obviously not that big, as Jerry has to pull himself along with his hands. His lack of vision reveals the darkness inside the tunnel.
After a while, we are told that the hole widens out, and he comes across crack running through the roof of the tunnel which allows sunlight in so that he can see the "clear jewel-green" colour of the water and the darkness of the tunnel ahead. The way that he is running out of breath in the final section means that we are not given an accurate account of the final stretch of the tunnel, but we know that the tunnel carries on in darkness until Jerry finally leaves it and is in the open sea.