These fish and their residual odors, however, could only have worsened the single most unpleasant aspect of U-boat life: the air within the boat. First there was the basal reek of three dozen men who never bathed, wore leather clothes that did not breathe, and shared one small lavatory. The toilet from time to time imparted to the boat the scent of a cholera hospital and could be flushed only when the U-boat was on the surface or at shallow depths, lest the undersea pressure blow material back into the vessel. This tended to happen to novice officers and crew, and was called a “U-boat baptism.” The odor of diesel fuel infiltrated all corners of the boat, ensuring that every cup of cocoa and piece of bread tasted of oil. Then came the fragrances that emanated from the kitchen long after meals were cooked, most notably that close cousin to male body odor, day-old fried onions.
All this was made worse by a phenomenon unique to submarines that occurred while they were submerged. U-boats carried only limited amounts of oxygen, in cylinders, which injected air into the boat in a ratio that varied depending on the number of men aboard. Expended air was circulated over a potassium compound to cleanse it of carbonic acid, then reinjected into the boat’s atmosphere. Off-duty crew were encouraged to sleep because sleeping men consumed less oxygen. When deep underwater, the boat developed an interior atmosphere akin to that of a tropical swamp. The air became humid and dense to an unpleasant degree, this caused by the fact that heat generated by the men and by the still-hot diesel engines and the boat’s electrical apparatus warmed the hull. As the boat descended through ever colder waters, the contrast between the warm interior and cold exterior caused condensation, which soaked clothing and bred colonies of mold. Submarine crews called it “U-boat sweat.” It drew oil from the atmosphere and deposited it in coffee and soup, leaving a miniature oil slick. The longer the boat stayed submerged, the worse conditions became. Temperatures within could rise to over 100 degrees Fahrenheit. “You can have no conception of the atmosphere that is evolved by degrees under these circumstances,” wrote one commander, Paul Koenig, “nor of the hellish temperature which brews within the shell of steel.”
The men lived for the moment the boat ascended to the surface and the hatch in the conning tower was opened. “The first breath of fresh air, the open conning-tower hatch and the springing into life of the Diesels, after fifteen hours on the bottom, is an experience to be lived through,” said another commander, Martin Niem?ller. “Everything comes to life and not a soul thinks of sleep. All hands seek a breath of air and a cigarette under shelter of the bridge screen.”
All these discomforts were borne, moreover, against a backdrop of always present danger, with everyone aware they faced the worst kind of death imaginable: slow suffocation in a darkened steel tube at the bottom of the sea.
On one of U-20’s patrols, this prospect came to seem all too real.
IT WAS EARLY in the war, when U-boat commanders and British defenders alike were developing new tactics to deploy against each other. Schwieger was scanning the sea through his periscope when he spotted two buoys ahead, spaced far apart. They had no obvious purpose, and their presence in that area of sea was unexpected.
Schwieger saw no danger. He called out, “Two buoys sighted. Keep exact depth.” The boat continued forward at “periscope depth,” 11 meters below the surface, about 36 feet, deep enough that only the top of the periscope showed above the water.
Something banged against the exterior, and then came a grating sound, like steel moving along the hull. “It sounded as if huge chains were banging against the boat and were being dragged over it,” said Rudolph Zentner, then on duty in the boat’s control room.
The men operating the ship’s horizontal rudders, the dive planes, called out in alarm. The rudders weren’t responding. Zentner checked the gauges that monitored depth and speed. The boat was slowing and sinking. It heaved and lurched from side to side.
Zentner watched the depth gauge and called each change to Schwieger. The boat sank deeper and deeper. At a depth of 100 feet, U-20 struck bottom. At this depth the pressure posed no threat, but the boat now seemed fused to the ocean floor.
Zentner climbed the ladder into the conning tower, and there looked out through one of the small windows of thick glass, the only means of observing the surrounding ocean while submerged. What he saw stunned him: a crosshatch of chain and cable. “Now we knew the meaning of those buoys,” he said. A giant steel net had been suspended between them, a submarine trap, and U-20 had run right into it. The boat lay on the bottom, not just ensnared but pinned down by the weight of the net.
And now, something else: through the walls of the hull the crew heard the thrum of propellers overhead. They knew from experience that this particular pattern of sound was generated by destroyers—“a shrill, angry buzz.” Depth charges did not yet exist, but the presence of destroyers waiting above was anything but reassuring. These were the ships that U-boat commanders most feared. A destroyer—a Donnerwetter—could move at 35 knots, or 40 miles an hour, and fire a lethal shot from a mile away. It could also kill a submarine by ramming. With a bow edged like a carving knife, a fast-moving destroyer could slice a U-boat in half.