Sending a mathematical message to the Yrr was the team’s first deliberate attempt to turn a crisis into a conversation. The plan joined a practical solution to the ocean’s communication barriers with a more speculative proposition: that mathematics might be intelligible across radically different forms of life. The signal was sent, but its reception and meaning to the Yrr remained uncertain.

The problem of speaking underwater

The contact strategy had been developed over several weeks by Samantha Crowe, with help from Murray Shankar, Judith Li, Leon Anawak, and NASA personnel. Earlier efforts at contact offered a point of comparison, but the underwater setting changed the problem. Crowe explained that water absorbs short-wave signals, while long-wave transmission would require enormous antennae. Light can carry information underwater only across relatively short distances. Sound was therefore the practical option, but it posed its own difficulty: echoes and reflections could make a message overlap with itself until it became unintelligible.

Leon Anawak described the team’s proposed workaround as an adaptation of marine-mammal communication. Dolphins modulate the frequency and pitch of their sounds; the team’s explanation was that this continual variation could reduce interference while increasing the amount of information transmitted. The device they planned to use was a singing modem. It could send thirty kilobytes over three kilometres, enough, Crowe noted, to transmit high-resolution images. The technical design was not incidental to the proposed contact: the team needed a signal that could survive the acoustic conditions of the water and carry more than a simple noise or pulse.

Mathematics as a first message

Crowe’s choice of content followed from her account of mathematics as a language grounded in physical law. She reasoned that a species arising under the same physical conditions might be able to recognize mathematical structure even if it shared none of humanity’s spoken languages or cultural conventions. The team would not begin by sending an account of human history or asking the Yrr to solve arbitrary sums. Instead, the first communication would be deliberately simple: two mathematical problems, intended to test whether the Yrr existed as an interlocutor and to gauge whether dialogue might be possible.

Crowe invoked the 1974 Arecibo transmission as an example of how a message could contain clues to its own interpretation. That message consisted of 1,679 characters encoded as binary pulses. Because 1,679 is the product of the prime numbers 23 and 73, a recipient who recognized that structure could arrange the pulses in 73 columns of 23 characters and recover a pattern. The example supported Crowe’s larger argument: a message need not share a language with its recipient if its organization offers a route to decoding it. The Arecibo signal, she pointed out, was expected to take 26,000 years to reach its destination, with a reply taking another 26,000 years. The underwater exchange, by contrast, could happen much more quickly.

The proposal also carried a specific uncertainty. The Yrr might already know something about humanity, but the team could not assume that they understood humans to be an intelligent species. The opening mathematical exchange was therefore meant not only to solicit a response but to establish what the senders believed to be the essential fact about themselves: that they possessed intelligence. Whether the Yrr would care to answer remained an open question.

Sending the signal

Late that afternoon, the team transmitted the first sound wave into the depths. Shankar selected a frequency matching the spectrum of unidentified sounds that his colleagues at SOSUS had named Scratch. The choice connected the new attempt to acoustic activity already detected in the ocean, without proving that the sounds had been made by the Yrr or that the Yrr would recognize the outgoing signal.

Reflection distorted the modem’s transmission. Crowe and Shankar worked from the CIC, modulating the modulations until they had reduced the distortion. About an hour after the broadcast, Crowe judged that any creature capable of detecting acoustic signals should be able to receive it. That assessment concerned the signal’s likely audibility, not its interpretation: the team still did not know whether the Yrr could make sense of the mathematical content. The act of sending thus produced a limited but consequential result. Humanity had placed a designed message into the sea; it had not yet established that another intelligence had understood it.

For Crowe, the transmission brought the long-imagined prospect of contact close enough to feel immediate. Her elation was inseparable from fear. Unlike Arecibo or Project Phoenix, this was not simply an experiment in communicating across distance: the team understood its work as part of an effort to avert catastrophe. The SETI researcher’s dream had acquired the burden of an emergency, and the first signal carried both the hope of dialogue and the responsibility attached to its possible failure.

What later biology clarified

Subsequent discussion of the Yrr’s biology supplied a striking comparison to the team’s sound-based approach, but not proof that the first mathematical signal had succeeded. Sue Oliviera described how the soil bacterium Myxococcus xanthus can respond to a chemical starvation signal: as more cells signal distress, the signal intensifies until the swarm gathers into a multicellular aggregate. In a later analysis of Yrr tissue, Oliviera identified a synthesized chemical pheromone associated with aggregation. In her account, this chemical works over short distances to summon free-floating amoebas, while the means by which the aggregated cells continue exchanging information remained uncertain.

Oliviera also explained that Yrr cells have paired receptors, which she interpreted as a system for distinguishing membership in the collective from fitness to participate in it. One receptor identifies a cell as Yrr; the other indicates that it is healthy and has intact DNA. According to her model, the pheromone can help exclude defective cells from aggregation and, when they are rejected, trigger their programmed death. These later findings make the contact attempt’s central challenge clearer: an intelligible mathematical message and the Yrr’s own chemical signaling belong to different communicative systems. The initial transmission tested whether a bridge could be made between them; the evidence presented here does not establish that the Yrr answered it.