This term refers to a malfunctioning torpedo that fails to hit its intended target and instead circles back toward the firing submarine. In the context of German submarines, such as U-boats during World War II, circle-runners posed a significant danger as they could potentially strike the submarines that launched them. The term highlights the risk inherent in submarine warfare of weapons unpredictably turning against their own operators. In the book’s Epilogue, Kurson shares his theory that a circle-runner may have caused the demise of the submarine at the dive site.

In deep-sea diving, decompression refers to the process of gradually returning to surface pressure after being exposed to increased pressure at depth. This is necessary to allow dissolved gases, particularly nitrogen, to safely exit the body and prevent decompression sickness, also known as “the bends.” Risks associated with decompression include the formation of nitrogen bubbles in the bloodstream and tissues if ascent occurs too rapidly, leading to symptoms of decompression sickness, including joint pain, paralysis, cardiopulmonary problems, and even death. Proper decompression procedures, including ascent rates and decompression stops, are crucial to mitigate these risks and ensure the safety of divers. The book describes how the highly experienced father-son dive team of Chris and Chrissy Rouse became disoriented and panicked on a dive as a result of nitrogen narcosis, surfaced without decompressing, and died of severe decompression sickness despite rescuers’ attempts to save them.