Imagine that you're at home, and it's quiet as a mouse. But outside, a big diesel truck is parked with its motor idling. The sound is barely audible, but once you notice it, it's distracting. If the truck were to stay there for days and weeks on end, the sound would spoil your concentration, even wake you up at night.
It could literally drive you crazy.
In the mountains of New Mexico, around the artists' colony in the little town of Taos, a few people report hearing just such a sound - all night, all day, every day.
Enough people that the sound has been given a name, "the Taos hum."
The American Southwest is famous for sightings of unidentified flying objects, outer-space creatures that have landed and conspiracy stories about top-secret government research. One wild theory blames the Taos hum on signals sent to submarines. Curious, since the closest deep water to Taos is more than 1,200 kilometers [770 miles] away.
So skeptics have dismissed the Taos hum as the product of vivid imaginations. After all, the vast majority of people in town don't hear a thing.
But scientists, including hearing specialists from the University of New Mexico, suspect something is going on. Perhaps what people are hearing is the common ringing sensation, called tinnitus, in their own ears. But tinnitus produces high-frequency sounds, and those who hear the Taos hum say it's low, down around the lowest C on a piano keyboard.
So the scientists planted listening devices around Taos that are thousands of times more sensitive than the human ear. But the needles haven't moved a hair.
So now the thinking is that hum hearers may not be picking up a single sound at all, but somehow accumulating traces of all the low, everyday sounds that surround us. This, of course, is little comfort to them.