We pulled off onto the gravel shoulder of Ranchlands Road, alongside the perimeter fence of the industrial park. We were about as close as we could get to the Grenadier factory without having an employee’s pass. I checked my mirrors for passing cars or onlookers and gave the all-clear. Stepping out of the car, I was immediately struck by the din coming from the factory; though I wasn’t quite sure the sound was low or reverberant enough to be our hum. Between the gravel shoulder and the fence was a shallow ditch choked with scrub grass and wildflowers. The cicadas screamed as we waded through the overgrowth, burrs sticking to our jeans, until we reached the chain-link fence on the other side.

Kyle removed his phone from his pocket and brought up his audio app. We stood quietly for a moment while he took a measurement. He squinted against the evening sun, small beads of sweat clinging to the translucent hairs above his lip. The air was thick and still. I looked out over the badlands that stretched beyond the industrial park, still untouched by developers. A landscape of horizons and subtle gradations of light. A landscape like an Agnes Martin painting, pure form and colour. The purple geometry of mountains in the distance, and lines receding into oblivion.

The cicadas are throwing it off, Kyle muttered. I noticed a car appear in the dancing heat to the south. I turned my body away from the road. Kyle did the same.

The factory had been on our list of possible sound sources from the start. Grenadier was a major defence contractor, manufacturing aerospace and military parts. I wasn’t exactly sure what kind of production activity went on there, but Kyle had read that some industrial furnaces can cause low-frequency rumbles and pressure waves. For now, it was as good a lead as any, even if it didn’t quite square with our hum, which, if anything, only grew louder at night, when the factory was closed. And it didn’t explain why we only began hearing the hum recently, despite the factory operating since before our neighbourhood was built. Nevertheless, we were too early in our mission to rule out any possibilities.

Kyle looked up from his phone. Seventy-eight hertz, he said. And thirty-two decibels, from about two hundred metres away.

That’s a pretty low frequency, I replied, recording the measurements in my notebook, beside the words ‘Grenadier plant, Ranchlands Industrial Park.’ But we both knew, without having to say it, that it was not low enough. Our theory—well Kyle’s theory, really—was that the hum must be an extremely low frequency, just on the threshold of human hearing. Most humans could just about register sound at twenty hertz. It seemed possible that we were simply hearing a sound lower than most people are able to perceive.