Doris Lessing builds suspense in "Through the Tunnel" by using words with negative connotations and painful imagery to describe Jerry's experience at the "wild bay." Instead of the "safe beach," where his mother sits, "a speck of yellow under an umbrella that looked like a slice of orange peel," Jerry's bay consists of "rough, sharp rock" and water with "stains of purple and darker blue," like bruises. He must run, "sliding and scraping" down the hill to the water, only to see "rocks [that] lay like discolored monsters" at the bottom. The stains sound like terrible bruises, the kind one might receive from coming into contact with the sharp rock formations; sliding and scraping makes one think of skin being scratched off knees or hands or elbows by rough stones. To then compare, via simile, the rocks under the water to monsters seems to be yet another sign that this is a dangerous place, and that Jerry is, perhaps, somewhat out of his depth (so to speak). These descriptions create a mood of foreboding and suspense as we wonder what other dangers he will encounter here.
Further, Lessing has Jerry count in order to keep track of how long he and the other, older boys stay under water before coming up for air. When Jerry is actually in the tunnel in the moments leading up to and during the climax, he counts, but, as he runs out of oxygen and begins to lose consciousness, he remains stuck on "a hundred and fifteen" for some time. He—and we—know this means we don't have an accurate idea of how long he's been down or how much longer Jerry can go before drowning, and this creates a great deal of tension and suspense for the reader.