In 1916, the first serious accident occurred that ever took place within the Danish submarine force, when the submarine DYKKEREN was sunk as a result of a collision. However, the subsequent successful rescue of the submariners trapped alive in the sunken submarine, which was the first rescue of its kind in history, was a superb achievement and a sensation that attracted worldwide attention.

The Practice Dive
On October 9, 1916, the coastal submarine DYKKEREN, accompanied by the 110 ton torpedo transport vessel SLEIPNER which was serving as a tender, had departed from Copenhagen and was preparing to make a practice dive in a training area to the east of Taarbaek Reef in the Sound, between Sweden and Denmark. Unfortunately, this training area was in the vicinity of heavily-traveled commercial shipping lanes, a situation which was exacerbated by the confined waters of the Sound. Aboard the submarine was a crew of 9 men, including its commander, Lieutenant Svend Aage Christensen.

At 1:40 p.m., the DYKKEREN commenced its dive, and disappeared beneath the surface at 1:45 p.m., while SLEIPNER marked the location where the DYKKERN's dive had taken place with red signal flags. When reaching periscope depth, DYKKEREN was to shape a north-eastern course.

However, no sooner had DYKKEREN submerged than the SLEIPNER observed a merchant steamer approaching the submarine, on a collision course.

The Collision
SLEIPNER urgently signaled the existence of an emergency situation with a series of rapid short blasts from its ship's whistle, instructing the Norwegian steamer VESLA, of Bergen, to bear to port, to avoid colliding with the submerged submarine. Only belatedly did the VESLA realize the danger and threw its engines into reverse, but it was too late. VESLA passed directly over the submerged DYKKEREN and a large volume of air bubbles was observed arising from the DYKKEREN's submerged location, indicating that the watertight integrity of the DYKKEREN's hull had been seriously damaged.

The time was now 2:02 p.m. SLEIPNER immediately proceeded at full speed toward the Danish gunboat GULDBORGSUND, anchored nearby, to raise the alarm. GULDBORGSUND immediately reported the situation to the high command of the Danish submarine flotilla at Copenhagen, which immediately made arrangements to send assistance. Prompt remedial action was to prove decisive.

VESLA's collusion had struck the submerged DYKKEREN's abaft her main hatch, puncturing her hull at this location. Seawater poured into the DYKKEREN's aftermost compartment, the battery compartment, which began to flood.

Trapped alive in a sunken Submarine
Inside the submarine, the DYKKEREN's commanding officer immediately blew the DYKKEREN's ballast tanks and ordered the drop keel released, in an attempt to obtain sufficient buoyancy to return the DYKKEREN to the surface. However, because the DYKKEREN was lying on the bottom at a 20-25 degree list, this attempt at damage control failed.

In the engine room, DYKKEREN's engineer sought to seal off the battery compartment by shutting the watertight door between the engine room and battery compartment. However, this attempt failed, with the result that the engine room also began to flood. Engine room personnel then retreated to the control room, where the rest of DYKKEREN's crew had also assembled, whereupon the engine room was sealed off by shutting its watertight door.

The Threat of Asphyxiation
The trapped submariners soon realized that they were faced with a entirely new and potentially fatal threat, this one from within the submarine. When the seawater flooding the submarine's battery compartment began to penetrate the battery cells and mixed with their content, a potentially fatal chlorine gas began to be emitted, fouling and poisoning their extremely limited oxygen supply within the diminutive, claustrophobic, 105 ton submarine.

Fortunately, even in the fledgling Danish submarine arm, the Danish navy had the foresight to provide the DYKKEREN with 12 life jackets equipped with a supply of oxygen and oxygen breathing gear. Unfortunately, 4 of these life jackets were located in the flooded battery compartment, and two of the remaining eight were destroyed by being contaminated by seawater. Thus facing certain death with the extremely limited air supply in their tiny submarine, the DYKKEREN's submariners had to act quickly if they were to save themselves.

Three Submariners' Desperate Escape
Time was short, and the situation extremely urgent. After holding an informal ship's council, the trapped submariners decided that the three crew members who had no life jackets, and thus no oxygen supply and oxygen breathing gear to protect themselves against the increasing volume of chlorine gas, should attempt to save themselves by escaping through the conning tower. Their situation of the men without emergency oxygen gear was desperate, and this improvised and risky means of escape was born of that desperation.

As the DYKKEREN was lying at a depth of only 4-6 fathoms, the water pressure was not great enough that the escaping submariners would be likely to suffer physical harm from decompression in the process of reaching the surface. The most likely risk would be in the desperate passage through the flooded conning tower. In any event, the risk had to be accepted, as otherwise the chlorine gas would soon prove fatal.

The escape plan was that the commanding officer (wearing emergency oxygen breathing gear) and one each of the three escaping crew members should enter the cramped conning tower, flood the conning tower and then open the outer conning tower hatch to allow the crew member to escape. After each man had exited the flooded conning tower, the commanding officer would then secure the outer conning tower hatch, and signal the men in the control room that the escape operation was concluded. The lower conning tower hatch would then be opened, and the conning tower drained of water. The entire operation would then be repeated two more times, until all three crewmen without emergency oxygen gear had passed through the cramped conning tower and escaped.

Triumph and Tragedy
The three submariners who had no emergency oxygen gear did in fact succeed in effecting their escape from the DYKKEREN according to their desperate plan and, remarkably, reached the surface successfully.

Unfortunately, during the course of rendering assistance to complete this series of successful escape operations to save his fellow crew members, the commanding officer, Sven Aage Christensen, tragically lost his life inside the darkened confines of the conning tower. Lt. Christensen probably lost consciousness after the last of the three submariners escaped from the DYKKEREN's conning tower, but before he could then save himself.

Lt. Christensen's body in the conning tower obstructed the passage of the remaining crew members to also effect their escape though the conning tower.

On the surface, the three submariners who had effected their escape were picked up by a boat from the Norwegian steamer VESLA. These lucky survivors were then transferred to the Danish gunboat GULDBORGSUND.

The Rescue Operation
After having raised the alarm about the DYKKEREN's accident, SLEIPNER returned to where DYKKEREN had sunk, and marked the spot with a dan buoy. Back in Copenhagen, the Danish navy had secured the assistance of the Danish commercial salvage firm Svister. By 3:30 p.m., Svister's rescue vessel KATTEGAT had arrived on the scene, and Svister's two lifting pontoons, the ODIN and THOR, were made ready to assist.

While the salvage vessels were being prepared, a diver was dispatched to the DYKKEREN with an air hose, which was connected to the DYKKEREN's torpedo room. Air could thereby be pumped into the torpedo room, where the DYKKEREN's remaining crew had in the meantime sought refuge, sealing themselves off from the remainder of the submarine by closing the torpedo room's watertight door.

The air pumped to the trapped submariners through the air hose relieved the acute air supply problem inside the DYKKEREN to the point that the remaining submariners could breath without utilizing their escape gear, and thereby conserved their remaining emergency oxygen supply in case of future need. By this means, crucial time was obtained in which to effect a salvage effort that might free the trapped crew.

Meanwhile, on the surface, the rescue party decided to attempt to free the five submariners remaining in the DYKKEREN by lifting the DYKKEREN's bow to the surface, thus enabling the trapped submariners to escape through the torpedo hatch, forward of the conning tower.

Two lifting cables were attached to DYKKEREN's hull, and the KATTEGAT proceeded to wench the DYKKEREN's bow to the surface. In time, KATTEGAT succeeded in raising DYKKEREN's bow to within one foot of the surface, the maximum the KATTEGAT could achieve under the circumstances. However, by redistributing the weight aboard the KATTEGAT, further leverage was placed on the DYKKEREN's hull and the torpedo hatch finally broke the surface and came clear of the water.

Sandbags were placed around the torpedo hatch to prevent the DYKKEREN from swamping when the hatch was opened. Finally, at 11:30 p.m. - not quite 10 hours after having been rammed and sunk - the torpedo hatch was opened and the remaining 5 submariners were freed. The commanding officer's body was also recovered from the DYKKEREN's conning tower.

Rewards, Including a Military Decoration
Because the DYKKERN's crew distinguished conduct and courage during their harrowing ordeal, they were all awarded three month's extra pay.

DYKKEREN's chief engineer, Christen Nikolaj Andersen, who assumed command upon the death of the DYKKEREN's commanding officer, Lt. Christensen, received special recognition for his skill and performance by being decorated with the Danish Medal of Honor.

The DYKKEREN's Fate
By the time the rescue of the DYKKEREN's remaining crew and the recovery of the commander's body was complete, the weather was deteriorating, so that any attempt to salvage the DYKKEREN herself had to be abandoned. The submarine was therefore lowered back to the sea floor. On October 13, 1916, the salvage vessels returned to complete their work, and the DYKKEREN was raised to a point which permitted her to be towed back to Copenhagen harbor, from whence she had departed only five days before.

A dockyard inspection of the salvaged DYKKEREN deemed this aged and obsolete submrine to be a constructive total loss, not worth the expense of repairing. DYKKEREN was therefore stricken from the Danish navy in June, 1917, sold, and scrapped in 1918.

Assessment
The Danes were extremely lucky that (1) the DYKKEREN sank in shoal waters, where she was easily accessible to salvage divers equipped with the relatively primitive diving gear of the day; (2) the diminutive DKKKEREN's submerged displacement was only a mere 132 tons, permitting her bow to be manipulated to the surface by the Danish salvage vessels; and (3) the collision had occurred within easily accessible range of the Royal Danish Dockyard at Copenhagen, which was thus able to render timely and effective assistance to rescue DYKKEREN's trapped crew - which otherwise faced almost certain death.

Under the foregoing extremely fortunate circumstances, though occurring in an unfortunate situation, the Danes had indeed been extremely lucky.

The DYKKEREN's sinking, and the ominous and harrowing entrapment of her crew under water, inside her sunken hull, was a wake-up call, not only for Danish submariners, but also for all naval services which employed submarines. Beneficial lessons were learned from the DYKKEREN's accident, which enabled the Danish navy to profit from the experience by instituting more effective damage control procedures and devising more effective emergency rescue gear and rescue arrangements for the future welfare and protection of Danish submariners.

In retrospect, the extremely favorable conditions of the DYKKEREN's loss, which greatly facilitated her crew's rescue, were unlikely to be replicated in the future, as submarine technology was constantly improving, along with the performance and operational capability of newer submarines. This meant that Danish submarines would be operating under increasingly demanding conditions, and that any future accidental submarine loss probably place far greater demands upon would-be rescuers.