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The safe beach is a place that Jerry associates with his mother, with childhood, and security. As he becomes more and more used to going to the wild bay, he feels that the safe beach "now seemed a place for small children [...]. It was not his beach." It has become boring, just as the trappings of childhood do for adolescents. Jerry is growing up.
The wild bay is a place that Jerry associates with independence, with masculinity, and adulthood. He sees older boys swimming there, boys who seem like "men to Jerry." They are capable of things that he is not, and he so desires to be accepted by them. This place is dangerous and lacks the security of the safe beach; here, Jerry is on his own and must rely on himself, not his mother, to make decisions.
The tunnel represents the passage from childhood to adulthood, from dependence to independence. In short, it is symbolic of growing up. Jerry cannot act childishly and successfully take on the task of swimming through the tunnel. Instead, he must practice and be patient. It is during this time that he develops a "curious, most unchildlike persistence." He must learn to delay gratification, but he must also be brave: two things that adulthood practically requires. Being an independent adult means taking some hard knocks, getting hurt (physically and emotionally) and facing risk. The tunnel is a powerful symbol because it seems to suggest a rebirth, the idea that Jerry will be transformed when he emerges on the other side. It's also powerful because this is a process we all go through, and so we can all connect to Jerry's ambivalence about becoming an adult as well as his overwhelming desire for independence.
In the short story "Through the Tunnel" Jerry is with his mother at the beach when he sees the other boys disappear. He later learns they have gone through an underground tunnel someplace else. He is curious where they have gone and wants to go. However, getting through the tunnel had proven difficult, so he prepares himself.
The safe beach is his childhood where his mother is present to nurture and protect him.
The wild beach is stepping out into the world. It is the risk that all adolescents take. He challenges himself just as boys do to become men.
The tunnel is the journey from childhood towards manhood.
The tunnel trip is the accomplishment that shows that he can over come adversity in order to win his role as a man. His mother will never be able to understand the significance of the trip.