The Earth’s geomagnetic field is extremely weak, around a hundred times smaller than that of a standard fridge magnet, and the solar and lunar tides are even tinier. Brown was one of the very first researchers to suggest that animals might have a magnetic sense, and he had no idea how his snails might detect such subtle changes.

But he knew it could be clinching evidence for his theory of external cosmic cues. He excitedly presented his results at the biological clocks symposium in 1960, telling the audience that living things are fantastically sensitive to very weak magnetic fields. Although we can’t see it, we’re all immersed in an electromagnetic ocean, he insisted, with waves, tides, and ripples that shift according to the relative positions of the Earth, sun, and moon, keeping organisms in constant touch with the state of the solar system and the time of day.

The bombshell didn’t convince his rivals, though; in fact it hardened them against him. Just as they were setting out rigorous principles for the study of biochemical internal clocks, the notion of a subtle magnetic sense was beyond the pale. And yet, though they shunned Brown in public, they didn’t completely ignore his idea: quite the reverse.

What’s rarely mentioned today about Aschoff and Wever’s famous bunker, built just a few years later, is that it contained not just one underground apartment, but two. The parallel units were almost identical, with matching beds, kitchens, and record players. But there was a very important difference: One of them was completely enclosed within a hefty capsule of cork, coiled wire, glass wool, and steel, through which no electromagnetic radiation could pass; anyone living inside was completely cut off from the Earth’s magnetic field. The aim was to show that the shielding made no difference to the volunteers’ biological clocks, and prove, once and for all, that Brown was wrong.

Between 1964 and 1970, more than 80 volunteers stayed in the two units. As Aschoff predicted, their circadian rhythms did continue. But there was a problem; the results in the two groups were not the same. In the unshielded bunker, isolated from clocks and sunlight but still exposed to magnetic fields, people’s sleep and waking patterns departed from the solar day, reaching an average period of 24.8 hours.

But when magnetic fields were also blocked, the volunteers’ circadian cycles deteriorated further. Their day length slipped even longer. There was significantly more variation between individuals. And their different rhythms were much more likely to become uncoupled. As mentioned earlier, Aschoff championed desynchronization as one of his key discoveries. Yet over those six years, it only ever occurred in the shielded bunker, cut off from the Earth’s magnetic field. Wever found that if he exposed the volunteers to a similar artificial field, all of these effects were reversed.

The results proved that we do have an inner clock that runs independently, regardless of any electromagnetic information from the outside world. And yet, that clearly wasn’t the whole story. Even though the volunteers couldn’t consciously perceive the Earth’s vanishingly weak magnetism, the results suggested that their bodies could somehow sense it, and that this had a profound impact on the workings of their biological clocks. Wever published the data in a series of now-obscure papers in the 1970s; it was a “remarkable” result, he said, the first scientific evidence that humans are influenced by natural magnetic fields. But Aschoff didn’t put his name to them, and he didn’t mention the shielding experiment or the apparent role of electromagnetic fields in any of his own papers on desynchronization.

The existence of the second chamber was largely forgotten, and circadian rhythm research continued as if the magnetism experiment never happened.

Just as these researchers were ignoring any links to magnetic fields, however, other biologists were being forced to address their effects, despite the dubious connotations. These scientists were studying the impressive ability of many animals to navigate across the planet, from turtles and salamanders to birds and bees. How did millions of monarch butterflies each year find their way thousands of miles from North America to a particular patch of fir groves in central Mexico?