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The underwater chorus of killer whales or dolphins could contain secret military messages hidden in plain sound. Chinese researchers have recently published a series of studies describing how to disguise underwater communications as artificial dolphin clicks and killer whale songs, potentially allowing stealthy submarines or underwater drones to cryptically pass covert military communications between each other or a home base.

Mimicking marine mammal sounds to disguise military communication is a decades-old idea that has resurfaced. During the Cold War’s geopolitical rivalry between the United States and the Soviet Union, the US military’s Project Combo tested using recordings of whale and dolphin songs as the basis for secret code that might go unnoticed by enemy eavesdroppers. But the Chinese researchers’ efforts seem to have gone a step beyond, by using modern technology to create artificial whale and dolphin sounds from scratch rather than relying on preexisting recordings.

“There have clearly been many recent advances in artificial signal synthesis,” says Kaitlin Frasier, an oceanographer and data scientist at the University of California, San Diego.

Advances in computing technology make it highly likely that the recent Chinese research efforts will find improved success in embedding useful information within the whale and dolphin sounds, Frasier says. “However,” she adds, “the technology is likely to run up against the same limitations that prior projects have encountered”—including limits on how far away such sounds can be detected, unwanted distortion of the signal caused by changing temperatures or other environmental factors that affect the bending and bouncing of sound waves, and interference from other ocean noises.

Sound is the ideal form of long-distance communication in the ocean’s dark depths, which limit the efficient transmission of light and radio waves. Whales and dolphins have naturally evolved to communicate over many kilometers, a fact that inspired the US Navy to launch Project Combo in 1959. In 1973, the navy successfully carried out tests between stationary underwater speakers and receivers at distances of up to 32 kilometers and depths of 75 meters. The following year, the submarine USS Dolphin used the technique to swap messages with a surface ship.

But the sensing and computing technology of the time faced limits in detecting the whale sounds without distortion, not to mention creating artificial whale sounds from scratch or decoding more complex meanings within artificially generated signals. These limitations meant that Project Combo’s experiments relied on prerecorded pilot whale sounds to build a predetermined code book with simple messages, rather than trying to synthesize customized messages on the fly. For their recordings, the navy settled on pilot whale sounds because of their efficient underwater transmission ranges, but also because the whales’ global presence meant they could potentially send messages around the world without arousing suspicion.

But modern technological advances in sensors and computing have allowed Chinese researchers at Harbin Engineering University and Tianjin University to potentially overcome some of those prior limitations. A long list of papers from both universities discusses analyzing and synthesizing the sounds from dolphins, killer whales, false killer whales, pilot whales, sperm whales, and humpback whales—all pointing to the possibility of creating artificially generated marine mammal sounds to send more customized messages.

“The idea is that you take the signal that the animal is making, and you are using it as your baseline signal,” says Roee Diamant, an electrical and computer engineer at the University of Haifa in Israel, who previously designed a covert underwater communication system for a defense technology company. On top of the marine mammal sound you add a modulation signal, Diamant says, which modifies the original sound to carry the coded message.

Transforming whale and dolphin sounds into a code that carries complex meaning for humans is not easy, Diamant says. Many whales produce sounds at lower frequencies, where the bandwidth available for constructing meaningful messages is limited. Sperm whales, however, communicate using clicks that can have wider bandwidth—but their short duration means the transmission energy, and therefore the possible distance of communication, is limited.

The dolphin family that includes killer whales and pilot whales transmits at higher frequencies, which may be more suitable for encoding messages. But Diamant and other experts suggest that any such effort would likely result in inefficient communications only suited for sending simple text-style messages.

“You’re not going to be transmitting books to one another,” says Ann Bowles, a senior research scientist focused on bioacoustics at the Hubbs-SeaWorld Research Institute in San Diego. “You could say ‘Go’ or ‘I’m here,’ fairly simple stuff like that, and likely get away with it—but if you’re trying to coordinate the movements of vessels or anything like that, it’s less clear.”

The Chinese researchers may find more success, Diamant says, in using marine mammal sounds not for secret communication, but to create covert sonar technology. Sonar works by sending out sound waves and listening for how those sound waves reflect off marine animals, the seafloor, or human-made objects such as ships and submarines. Chinese teams have proposed substituting a natural sound like a sperm whale click—the whale’s own biological sonar in the form of echolocation—for sonar sound waves, so that intended targets don’t realize they’re being tracked. “If you need to transmit a signal, you might as well use an emission used by a marine animal,” Diamant says.

Of course, there is one potential problem: no one knows how this kind of covert communication system—whether echolocation or vocalizations—could affect actual marine mammals. Would they be confused?

Previous experiments suggest whales and dolphins can quickly figure out when the sound being played is a recording or otherwise artificial, but they may still change their behavior in response by increasing and changing their own vocalizations, Bowles says.

“Dolphins are known to whistle back in response to signals in their whistle frequency band, and we have known them to actually jam those frequency bands with intense whistling in response to a stimulus,” Frasier says. “Dolphins, including pilot whales, are generally quite curious, so it’s likely that they would be intrigued by and respond to these signals.”

That could mean negative impacts for whales or dolphins, if their sudden flurry of communication in response to artificial noise gives away their own location to predators and prey. Artificial signals could also cause them to abandon their current plans and move away from the noise. At the very least, the system could distract the animals and waste their time by forcing them to figure out what’s going on. But if it’s a rarely used system, such impacts could be minimal compared with the many different underwater sounds that marine mammals already deal with on a daily basis.

“In [the United States], if we were to use such a technique, there would have to be experimental proof that the challenge posed was not overwhelming for any other species in the area where you were planning on using it,” Bowles says.

The Chinese research does not appear to have progressed beyond the laboratory stage yet, and it’s unknown if the US Navy has continued Project Combo. Laws such as the Marine Mammal Protection Act of 1972 have given rise to regulatory efforts aimed at controlling the amount of noise humans generate in the oceans, and military organizations such as NATO have standards for underwater sounds. But a military could still conduct such experiments secretly without notifying anyone, Diamant points out: “Just because you can’t read about it anymore doesn’t mean they don’t do it, right?”