Recording infrasound on land isn’t particularly tricky; you can place sensors pretty much anywhere. Not so in the oceans of the Southern Hemisphere: Sensors can only be placed on mostly small, lonely islets, so the coverage is poor.
And, den Ouden says, out in the open ocean the “huge chaos of waves” makes a lot of undesired noise. Some of this irritating infrasound comes about as the sloshing sea surface waves interact. “The ocean starts to go up and down with a rhythm,” says den Ouden. The sea acts like a gigantic speaker, blasting energy into the atmosphere that travels upwards and across the water, toward the land, like an invisible tidal wave. Other oceanic infrasound is less problematic but more mysterious: The motion of the sea triggers atmospheric vibrations that radiate straight upward. But these waves have proven so difficult to detect that their existence has long been an open question.
This collection of infrasound waves, which are technically known as microbaroms, have been referred to as the “voice of the sea.” Most researchers want to drown it out. “We try to get rid of the microbarom signal, because we’re interested in explosions,” Iezzi says.
Ideally, infrasound detectors at sea would not only be able to fill in a vast coverage gap, but also document the microbaroms well enough that, with the help of filtering software, they could be effectively canceled out. But where would you put these detectors? Boats wouldn’t work. “The problem with them is that they’re moving up and down all the time,” says Lamb—and that would mess with the recording. Balloons have been used to record infrasound on land, but their flight paths over the sea would be too unpredictable to be of any use. (They would, however, be useful for recording lightning strikes, quakes, and volcanic eruptions on Venus, because the surface of Earth’s evil twin is so hot that any instruments placed on the ground there would quickly melt. Or, at the very least, overheat.)
The open ocean is “an extremely challenging place to record sound,” says Bowman, “so challenging, in fact, that if you’d asked me prior to looking at this paper, I’d have said it’s basically impossible.”
As it happens, Samantha Patrick, a seabird ecologist at the University of Liverpool, was curious about the ability of seabirds to navigate using infrasound. After conversing with den Ouden and his weather- and geophysics-focused colleagues, they developed an outré idea: Why not attach microbarom detectors to birds? And not just any birds: wandering albatrosses. Their wingspans, which can be 11 feet long, are lengthier than any human is tall. This allows them to spend considerable time simply floating on air currents above open waters, something that conserves energy as they embark on foraging trips. Not only do they fly across vast swaths of isolated ocean, but they don’t dive into the water, so any sensors attached to them wouldn’t get especially wet.
In short order, the researchers built minuscule infrasound sensors and fitted them into pouches—packages no more hefty than a TV remote. As fun as it may be to visualize these bags being lugged about the way a school kid carries a backpack, that would have been needlessly complicated. Instead, the pouches were simply stuck to the backs of the avian assistants with some duct tape.
Last year, the team headed to the Crozet Islands, little blips of land in the French sub-Antarctic on which wandering albatrosses like to nest. But how, pray tell, do you get albatrosses to cooperate? With a very special sort of hug, apparently—one that prevents any potentially injurious flapping and pecking. “They don’t really have predators—certainly no natural predators,” says Patrick, who assisted the team with their research. “So you literally just walk up to it, and then you put your hand on its bill, and then you have to hug it, because it’s so big. You give it a hug and lift it off the nest, and then one person holds it, and then the other person duct-tapes the logger to their back.”