As search teams were continuing to search for the missing ARA San Juan submarine Thursday, a Vienna-based organization announced that its scientists had detected an “unusual signal” underwater in the proximity of the area where it had gone missing days earlier.

The Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization usually focuses on monitoring possible violations of a treaty, which has 183 member states and was negotiated in the 1990s. The post-Cold War agreement bans nuclear explosions, even though key signatories such as the United States never ratified it.

When its researchers noticed an unusual spike in sound waves at the end of last week, however, they concluded that the incident was unlikely due to a nuclear test — but still had unnatural origins. The sound waves were registered only hours after the Argentine submarine disappeared and could be tracked back to the area where the missing crew had last sent a communication.

“It could be consistent with an explosion but there is no certainty about this,” Mario Zampolli, a hydroacoustic engineer with the international nuclear test-ban body, told Reuters on Thursday.

Since 1996, the international body with its staff of more than 260 employees and an annual budget of about $130 million has established a network of 15 certified monitoring stations and certified about 300 more, which are operated externally. Its underwater stations are usually 2,000 feet to 0.7 miles below sea level and are mostly equipped with high-tech microphones, which can detect unusual sound waves. Thursday's announcement was based on signals detected by two of those monitoring points. The first station recorded the initial signal in the Atlantic Ocean, whereas the second station in the Indian Ocean was believed to have registered a sound echo much farther away from the point of origin.

An undated photo made available by the Argentine navy Nov. 17 shows the submarine ARA San Juan. (Argentine navy/European Pressphoto Agency/EFE/REX/Shutterstock)

Sound waves from explosions or similar events can travel thousands of miles through the ocean.

“Due to the efficient transmission of sound through water, even comparatively small signals are readily detectable at very long distances. Thus, eleven stations are sufficient to monitor the Earth’s big oceans, with emphasis on the Southern Hemisphere which is largely dominated by water,” the scientists wrote on their website in a general description of their work. The group's mission also includes monitoring possible treaty breaches on land, where it relies on devices, which are able to detect subsonic noises, spikes in radioactive particles or seismic vibrations.

When the first monitoring station in the Atlantic Ocean detected a possible explosion last week, the group transmitted the data to its IT center in Vienna. On their website, the researchers describe how they detect abnormalities in the data, both through algorithms and — if an event is automatically flagged — through researchers' intervention.

That process can be lengthy, which may explain why the “unusual signal” was not immediately flagged to authorities in Argentina.

Although Argentine authorities later described the signal as an “explosion,” the Vienna-based researchers' statement and a broader online explanation of their work indicates that such an assessment may not be fully reliable, based on the data that can be collected from underwater microphones.

The organization's researchers can only distinguish between three categories of signals, with “in-water explosions caused either by man-made devices or underwater volcanic eruptions” falling into the first.

Earthquakes produce the second distinguishable sound, whereas “noise signals from a wide range of physical sources such as iceberg-generated noise, airgun surveys and whale song” belong into a third category.

Hundreds of events are analyzed by the group's staff every day, and in the 20 years the organization has operated, it has also helped to monitor potential tsunamis and the fallout of the Fukushima nuclear accident in 2011. The organization has become a crucial global monitoring center for all kinds of incidents, but since 1996, only three countries have violated the nuclear test-ban treaty, which led to its creation: India, Pakistan and North Korea.

Read more: 

Sound ‘consistent with an explosion’ heard near missing Argentine sub’s last known location