• A secret network of underwater microphones heard a sound suspected to be from the moment the Titan submersible imploded, crushed by pressure from the outside.
  • The Navy immediately reported the sound to searchers.
  • It was considered only evidence until searchers found the debris field from the missing sub.

The U.S. Navy detected a sound in the north Atlantic Ocean on Sunday, June 18, believed to be the moment the submersible Titan was destroyed in an implosion. The sound was recorded on a secret underwater surveillance network designed to detect enemy submarines, but it was considered just a piece of evidence until the U.S. Coast Guard announced on Thursday that remotely operated vehicles had discovered a debris field near the Titanic wreck, matching the description of the lost submersible.

A defense official told the Wall Street Journal that the Navy had started listening for the submersible as soon as it lost communications with its mothership 1 hour and 45 minutes into a mission to explore the Titanic wreck site. “The U.S. Navy conducted an analysis of acoustic data and detected an anomaly consistent with an implosion or explosion in the general vicinity of where the Titan submersible was operating when communications were lost,” a senior Navy official said.

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The U.S. Navy’s ocean surveillance system is used to track Russian submarines, especially the Yasen class, seen here, capable of launching nuclear-tipped cruise missiles at the Eastern Seaboard.
Getty Images

The Navy then passed on the information to the “commander on site,” which would mean the Coast Guard commander in charge of the overall rescue effort. Between the sound and the debris field, the two pieces of evidence were considered enough for the Coast Guard to declare the sub lost.

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But how did the Navy hear the underwater implosion in the first place?

The U.S. officials quoted are referring to the Integrated Undersea Surveillance System (IUSS), a worldwide network of underwater sensors designed to detect, classify, and report the location of submarines worldwide. The network largely consists of underwater microphones, known as hydrophones, that listen for submarine activity. The system was scaled back with the breakup of the Soviet Union in 1991, reduced from 30 undersea surveillance sites to just three in 2016.

In addition to static hydrophones, IUSS consists of a small fleet of ships that ply the oceans, trailing similar listening hydrophones. The ships are equipped with the Surveillance Towed Array Sensor System (SURTASS), which includes both listening gear and an “active” sonar system that sends low-frequency pulses meant to bounce off enemy subs.

The Navy’s statement implies that IUSS has a long rolling record of underwater sounds, and that the Navy can “scroll back” in time and search for them. It can also geolocate the location of the sound, a logical capability since a long record of underwater sounds is worthless if you don’t know where they were heard.

090309 n 0000x 001washington march 9, 2009 the military sealift command ocean surveillance ship usns impeccable t agos 23 is one of five ocean surveillance ships that are part of the 25 ships in the military sealift command special mission ships program impeccable directly supports the navy by using both passive and active low frequency sonar arrays to detect and track undersea threats us navy photoreleased
USNS Impeccable is one of several ships that make up the IUSS.
U.S. Navy Photo

Due to the sensitive nature of the data collected, the exact details of the IUSS is a tightly held secret. In 2009, Chinese ships harassed one of the SURTASS ships, the USNS Impeccable, as it sailed within 75 miles of Chinese territorial waters.

The Navy isn’t the only organization with this capability. The Comprehensive Test Ban Treaty Organization (CTBTO), an international organization set up to monitor the entire planet for signs of nuclear weapons tests, once detected the breakup of a missing submarine. The CTBTO maintains a global network of sensors, known as the International Monitoring System, which includes 11 hydroacoustic sensors. In 2018, the underwater sensors picked up the destruction of the Argentine submarine ARA San Juan from up to 8,000 kilometers away.

Headshot of Kyle Mizokami
Kyle Mizokami

Kyle Mizokami is a writer on defense and security issues and has been at Popular Mechanics since 2015. If it involves explosions or projectiles, he's generally in favor of it. Kyle’s articles have appeared at The Daily Beast, U.S. Naval Institute News, The Diplomat, Foreign Policy, Combat Aircraft Monthly, VICE News, and others. He lives in San Francisco.