One of the themes in "Through the Tunnel" highlights the developing individualism of a young boy. To that end, the setting in Lessing's story contributes to the development of this theme.
Setting is defined as the place, time, and/or duration of important events in a story which culminate in a resolution; setting is often conveyed to readers through the use of rich imagery.
In the story, Jerry is vacationing at the beach with his mother. Even from the very beginning, Jerry wants to go off by himself to 'have a look at those rocks down there.' He wants to explore the area by himself, to assert his growing curiosity about the world of his childhood vacations. His mother thinks that the area where he wants to explore is 'a wild-looking place' with no visible vacationers in sight, but this is precisely what Jerry likes about the place: it is unknown, vast, and possibly even dangerous. In fact, it is a 'steep descent to the bay.'
We feel Jerry's sense of accomplishment as he swims past a 'middle region where rocks lay like discolored monsters under the surface' to arrive at 'the real sea, a warm sea where irregular cold currents from the deep water shocked his limbs.' The irregular, cold currents represent uncertainty; even the rocks look surreal, perhaps concealing latent, undefined threats to this courageous adventurer. Lessing cleverly uses the imagery of setting to highlight both the euphoria and the emotional conflict present in her young protagonist as he breaks away from his mother to explore his surroundings. The 'break' is as emotional as it is psychological. Even Jerry's mother senses that this 'break' is necessary to her son's maturing individuality: 'she was determined to be neither possessive nor lacking in devotion.'
Lessing further carries this theme of individualism into subsequent images of another setting. This time, Jerry comes across a group of boys who are 'diving again and again from a high point into a well of blue sea between rough, pointed rocks.' Apparently, this individualism carries with it a high price. Jerry must and wants to earn his right to swim with the older boys. However, he soon discovers that he must conquer the long, dark, and ominous tunnel under a great barrier rock if he wants to accomplish the same swimming feat as the bigger boys. Once again, the imagery presents an imposing portrait of challenge and peril characteristic of a theme which explores courageous individualism.
Hope this helps!