Recording infrasound on earth is not particularly delicate; you can place sensors just about anywhere. This is not the case in the oceans of the southern hemisphere: sensors can only be placed on small, mostly isolated islets, so coverage is poor.
And, says den Ouden, in the open sea, the “enormous chaos of waves” makes a lot of unwanted noise. Some of this irritating infrasound occurs when sea surface waves interact. “The ocean begins to rise and fall with a rhythm,” explains den Ouden. The sea acts like a gigantic loudspeaker, projecting energy into the atmosphere which rises and crosses the water, towards the land, like an invisible tidal wave. Other oceanic infrasounds are less problematic but more mysterious: the movement of the sea triggers atmospheric vibrations that radiate directly upwards. But these waves have proven to be so difficult to detect that their existence has long been an open question.
This collection of infrasound waves, which are technically known as microbaromes, have been called the “voice of the sea”. Most researchers want to drown it. “We’re trying to get rid of the microbarom signal because we’re interested in explosions,” Iezzi explains.
Ideally, infrasound detectors at sea would not only be able to fill a large coverage gap, but also document microbaromes sufficiently so that with the help of filtering software they can be effectively canceled. But where would you put these detectors? The boats would not work. “The problem with them is that they go up and down all the time,” says Lamb, and that would mess up the recording. Balloons have been used to record infrasound on land, but their flight paths over the sea would be too unpredictable to be useful. (However, they would be useful for recording lightning, earthquakes, and volcanic eruptions on Venus, as the surface of Earth’s evil twin is so hot that any instrument placed on the ground there would melt quickly. Or, to at the very least, overheat.)
The high seas are “an extremely difficult place to record sound,” Bowman says, “so difficult, in fact, that if you had asked me before reading this article, I would have said it is basically impossible.
As it turns out, Samantha Patrick, a seabird ecologist at the University of Liverpool, was curious about the ability of seabirds to navigate using infrasound. After talking to den Ouden and his weather and geophysical colleagues, they came up with an outraged idea: why not attach microbar detectors to birds? And not just any birds: howler albatrosses. Their wingspans, which can be 11 feet long, are longer than any human. This allows them to spend a considerable amount of time just floating on air currents above open water, saving energy when they embark on foraging trips. Not only do they fly over vast expanses of isolated ocean, but they don’t dive into water, so sensors attached to them wouldn’t get particularly wet.
In no time, the researchers built tiny infrasound sensors and put them in pouches, packages no heavier than a television remote control. As fun as it might be to visualize those bags being dragged along like a schoolboy carrying a backpack, it would have been unnecessarily complicated. Instead, the pouches were simply stuck to the backs of the avian helpers with duct tape.
Last year, the team visited the Crozet Islands, small pieces of land in the French sub-Antarctic on which howler albatrosses like to nest. But how, please, do you get the albatrosses to cooperate? With a very special kind of hug, apparently, that prevents any potentially dangerous flapping and pecking. “They don’t really have predators, certainly no natural predators,” says Patrick, who helped the team with their research. “So you literally walk up to him, then you put your hand on his beak, then you have to give him a hug, because he’s so big. You give it a hug and lift it off the nest, then one person holds it, then the other person taps the lumberjack behind their back.