"Through the Tunnel" is a coming-of-age story of sorts, and as such, one of its primary themes is the innocent child coming to terms with his own mortality. Therefore, there could hardly be a better setting than the open water, a wondrous but dangerous space where children can both play and be in extreme peril.
Even before we see our protagonist getting himself into dangerous situations, Lessing gives us the following description of the sand and the water:
He went out fast over the gleaming sand, over a middle region where rocks lay like discolored monsters under the surface, and then he was in the real sea—a warm sea where irregular cold currents from the deep water shocked his limbs.
Although this quotation gives us a sense that the beach is quite beautiful, it also comes off as rather frightening. After all, the rocks are described as "discolored monsters," and the water rather violently interacts with Jerry's body by shocking his limbs.
This is not the only quote we are given...
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