The diesel-electric ARA San Juan was returning to its base south of Buenos Aires after a routine mission to Ushuaia, near the southern tip of South America. Then, suddenly, it went silent.
According to The Associated Press, no one has been able to contact the sub or any of its 44 crew members since Wednesday, even though an international collection of rescuers are scanning all radio frequencies and scouring the waters near the San Juan's most recent ping.
Complicating matters: strong winds and high waves that were battering search-and-rescue ships.
The Argentine government had received logistical help from the governments of Britain, Chile and the United States, including NASA - other countries have also offered aid - but as of Saturday morning, no surface or visual contact had been made, the AP reported.
The sub has multiple ways of communicating. It has ample food and oxygen, the Argentine navy said, and its protocol is to surface if there's a communications blackout.
"The last position [registered] was two days ago," navy spokesman Enrique Balbi said, according to the AP. "Without wanting to be alarmist or overdramatic, the facts are that no form of communications could be established between the vessel and its command, even with the alternative methods that the submarine has.
"What we interpret is that there must have been a serious problem with the communications [infrastructure] or with the electrical supply, cables, antennae or other equipment."
Worried relatives had gathered at the submarine's base, where they hoped to hear the first updates.
"We are praying to God and asking that all Argentines help us to pray that they keep navigating and that they can be found," Claudio Rodriguez, the brother of one of the crew members, told the local Todo Noticias TV channel, according to the AP. "We have faith that it's only a loss of communications."
News of the stricken submarine had even reached the Vatican. Pope Francis, an Argentina native and the former archbishop of Buenos Aires, offered his "fervent prayers for the 44 officers aboard the ARA San Juan" in a message released on his behalf Saturday by Cardinal Pietro Parolin, the Vatican's secretary of state, according to CNN. Francis "asks that his closeness be conveyed to their families and to the military and civil authorities of the country in these difficult moments."
Those family members and the Argentine government were facing a cruel fact of submarine life. The vessels are often among a country's most expensive and complex military assets - and, during accidents or times of crisis, their most vulnerable.
Over the years, several submarines have vanished, often igniting mysteries that lasted decades.
On May 27, 1968, the USS Scorpion failed to return to port, unexplainedly sinking 11,220 feet beneath the Atlantic Ocean along with its 99 crewmen and two nuclear torpedoes, according to USA Today. A Navy inquiry found that the cause of the loss "cannot be definitively ascertained" - and the cause of the sub's demise still remains fuzzy decades later.
Theories abound of course: a torpedo self-fired into the ship, destroying it from the inside, or a battery exploded, inflicting critical damage. The Navy has routinely tested the water around the ship for radioactivity, according to USA Today, but has denied a proposal by civilian marine disaster experts to investigate the wreckage.
In August 2000, the Russian nuclear submarine Kursk suddenly sank during a planned and closely-monitored Russian military exercise, killing all 188 sailors aboard, according to The New York Times. It was hours before the Russian government even knew something was amiss.
The most likely explanation was that fuel in a torpedo detonated, setting off a chain reaction in a sub once deemed unsinkable. The Russians have said the Kursk used an outdated and unstable hydrogen peroxide propellant.
Conspiracy theories abound, and at least one real-life horror story was verified: Not all of the sailors died in the initial blast, according to the Times.
For hours, some fought fruitlessly to survive.
"13:15," Lt. Capt. Dimitri Kolesniko, the commander of the turbine room wrote, noting the military time. "All personnel from compartments six, seven and eight moved to the ninth. There are 23 of us here. We have made this decision as a result of the accident. None of us can get out."
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