Read the excerpt from The Dark Game: True Spy Stories from Invisible Ink to CIA Moles . Wyke's work was always delicate and demanding, more so in the damp and confined space. Gently and carefully, Wyke scraped the dirt until he found the three cables encased in black rubber and, he believed, pressurized by nitrogen, a common practice in the 1950s to keep moisture out of cables. He knew that once he cut through the rubber sheathing on the cables, the nitrogen would escape. The resulting drop in pressure would be detected by the Soviets. The engineers solved this potential problem by building a concrete barrier with a steel door to pressurize the tap chamber and separate it from the pre-amp room. (It turned out that the cables were not pressurized.) Wyke next attached wires to the exposed cable—probably with alligator clips—and began to draw power from the cables. Once again, the work called for a deft hand. If he drew off too much power, the Soviets would be sure to notice it. As expected, Wyke did a masterful job, and the tapping began on May 11, 1955. Which sentence best states the controlling idea of the passage?