What if, out of nowhere, you started hearing a low-frequency rumble, like a truck idling right outside your home, in the middle of the night, every night for weeks and months on end -- and no one else could hear it?
This is the case for thousands of people around the world. Out of the blue, this unexplained sound becomes an inescapable part of nocturnal life. Those who hear it are compelled to hunt for its source, but to no avail. Between being kept awake by the unrelenting hum, and doubting one's sanity, the experience can take a toll on both physical and mental health.
Glen Macpherson has created a global database on the "Worldwide Hum," with thousands of people mapped across five continents. And now, he thinks he might be on the cusp of figuring out what's causing it.