Infrasound recording on the ground is not particularly difficult; You can place the sensors almost anywhere. It is not so in the oceans of the Southern Hemisphere: the sensors can only be located on small, often lonely islands, and therefore coverage is poor.
As Den Auden says, in the open ocean, the “great chaos of the waves” makes a lot of unwanted noise. Some of this disturbing infrasound comes from when the flowing sea surface waves interact. “The ocean starts to go up and down with a rhythm,” says Den Auden. The sea acts as a giant spokes, spewing energy into the atmosphere that travels up and through the water, toward the land, like an invisible tidal wave. Other oceanic ultrasounds are less problematic but more ambiguous: the motion of the sea causes atmospheric vibrations that radiate directly upward. But detecting these waves has proven so difficult that their existence has long been an open question.
Technically known as microbaroms, this group of infrasound waves has been referred to as the “sound of the sea.” Most researchers want to sink it. “We’re trying to get rid of the microbarome signal, because we’re interested in bangs,” Azzi says.
Ideally, infrasound detectors at sea would not only be able to bridge a wide coverage gap, but would also document the microbarum well enough that, with the help of filtration software, it could be effectively cancelled. But where will you put these reagents? The boats will not run. “Their problem is that they go up and down all the time” — and that would ruin the recording, says Lamb. Balloons were used to record ultrasound on land, but their flight paths over the sea would be too unpredictable to be of any use. (They will, however, be useful for recording lightning strikes, earthquakes, and volcanic eruptions on Venus, because the surface of Earth’s evil twin is so hot that any instruments placed on Earth would quickly melt. Or at least, heat up.)
The open ocean is “a very difficult place to record sound,” Bowman says, “It’s very difficult, in fact, if you had asked me before you looked at this paper, I would say it’s basically impossible.”
As it happens, Samantha Patrick, a seabird ecologist at the University of Liverpool, was curious about seabirds’ ability to navigate using ultrasound. After speaking with den Auden and colleagues interested in weather and geophysics, they developed an unexpected idea: why not attach microbarome detectors to birds? And not just any birds: the wandering albatross. Its wingspan, which can be up to 11 feet long, is longer than that of any human. This allows them to spend a long time simply floating on air currents over open water, something that conserves energy while embarking on foraging expeditions. Not only does it fly across vast stretches of isolated ocean, but it doesn’t dive into water, so any sensors specifically attached to it won’t get wet.
In a short time, the researchers made small infrasound sensors and put them in bags – packages no larger than a TV remote control. As fun as it might be to imagine these bags being pulled around the way a school kid would carry their backpack, it was unnecessarily complicated. Instead, the bags were simply glued to the backs of the bird’s helpers with some masking tape.
Last year, the team headed to the Crozet Islands, tiny patches of land in the French sub-Antarctic where wandering albatrosses love to live. But how do, let’s say, get the albatrosses to cooperate? With a very special kind of hug, apparently – preventing any flickering and malicious clicking. “They don’t really have predators — and they certainly don’t have natural predators,” says Patrick, who helped the team with their research. “So you literally walk up to her, and then you put your hand on her bill, and then you have to hug her, because she’s so big. You hug him and you lift him out of the nest, and then someone grabs him, and then the other person straps the recorder on his back.”