The deeper meaning of the story "Through the Tunnel" is the boy is growing up and pulling away from his mother. To do so he tests himself to see if he can make it through the tunnel. He does not realize that is what he is doing. He sees the other boys going through it and he is curious. Each time that he tries to get through he does not make it.
The boy decides that he needs to give the tunnel one more try. He knows if he does not try it that he will never return to trying it and he will have given up. It is very difficult but he makes it. He hangs out and dives and swims on the other side where the other boys are for awhile and then he makes it back.
The boy has done something that his mother will never be able to understand the level of his accomplishment. He has also taken the first step towards adulthood.