Maybe you hear it. A low frequency hum, almost a vibration, just on the threshold of human hearing. Maybe it keeps you awake. Maybe it causes you headaches, dizziness, even nosebleeds. If you do hear it, you're among the roughly 4% of the world's population affected by the Hum", a frequently reported but little understood global phenomenon. The earliest reliable reports of the Hum date from the UK in the mid-1970s. Numerous reports of the Hum have been made across the UK, usually clustered around specific towns or cities: Hythe, Plymouth and, as recently as last month, Swansea. The fact that the Hum seems to have only really emerged as a documented concern in the past half-century suggests it could be a byproduct of technological advances. As much as our innovations have the capacity to nurture and sustain us, they also have the capacity to assail us. It always comes as a small surprise to remember we are constantly beset by high- and low-pitched frequencies, which our brain actively tunes out. Could the Hum be the background thrum of electricity, gas lines or cell towers? One theory even posits ultra-low frequency radio signals used to communicate with submarines in the depths of oceans might be interacting with soft tissue in our skulls that stimulate the auditory nerve " a phenomenon known as the "microwave auditory effect", which, incidentally, has been studied by the Pentagon for use as a sonic weapon.