One theme of Doris Lessing's short story, "Through the Tunnel," is that growing up is a difficult and sometimes painful process. We see Jerry mature throughout the story, at first nagging and pestering his mother for goggles and later being able to delay gratification for the things he wants. He practices holding his breath with very adult diligence, he stops asking permission to go to his "wild bay," and, the narrator says,

A curious, most unchildlike persistence, a controlled impatience, made him wait. 

Although he believes it is likely that he could make it through the tunnel on this day, Jerry waits. He can now wait to gratify his wishes, something associated with emotional maturity. However, after he successfully swims through the tunnel the next morning, his feelings have changed. He could see the boys he so wanted to impress earlier in the week, the boys who seemed like men to him, but now,

He did not want them. He wanted nothing but to get back home and lie down.

He has endured several bloody noses, and now he has a banged up head. In fact, Jerry nearly died in the tunnel and felt certain that his eyeballs had burst when he finally reached the water's surface. In the end, his mother tells him not to swim anymore today, and though "she was ready for a battle of wills . . . he gave in at once."

We see then that Jerry has, in fact, matured as a result of his experiences, and that they are symbolic of a child's transition to adulthood.  It is an arduous and taxing process, one that is difficult for both parent and child. 

There are at least three themes in "Through the Tunnel":

• Maturation 

In the beginning of the narrative, the eleven-year-old Jerry is attached to his mother, watching for the "yellow speck" on the beach under the orange umbrella, even when he swims with the native boys on the wild bay.  But, when they disapprove of antics that have previously humored his mother, Jerry realizes he must assert his independence from his her. So, he swims to shore and demands swim goggles, nagging and insisting until he procures them. Later, when he desires the acceptance of the older boys and wants to feel mature himself, Jerry swims in the bay independently, and he practices going under water so that he can be like them.

• Rite of Passage

Sensing that the older boys find him childish because they can do things he cannot, Jerry practices until he, too, can pass through the long underwater tunnel. When he finally attempts to pass through this tunnel, Jerry senses that his life is in danger because his lungs ache. Nevertheless, he perseveres,

He struggled on in the darkness between lapses into unconsciousness....His hands, groping forward, met nothing....

Finally, Jerry reaches the surface, "gasping like a fish." Successful, he knows he has matured.

He did not want them.  He wanted nothing but to get back home and lie down.

• Alienation

Apparently without a father, Jerry spends most of his time with his mother. At least on this vacation, Jerry lacks friends, and his attempt to impress the native boys seems to indicate that Jerry is often alone. At the end of the story, more confident with himself, Jerry does not seem to mind being alone, however.