“Nightline” aired a piece on Friday about a mysterious, low-frequency hum that afflicts many residents of the small village of Woodland in County Durham, England, every night. Some of the residents thought they were going mad, until they discovered they weren’t the only ones who had heard the hum.

Reports of the strange humming noise has circulated the globe, popping up in New Zealand, Canada, the United States and of course several towns in England. Watch ABC’s Nick Watt’s report here:

We asked viewers to tell us what they think is causing the strange hum. Here are some of their answers:

“Here in Warrenville, IL we hear a hum late at night that we attribute to activity at Fermilab. Since there is no Fermilab in England, I would think it is a high voltage transformer somewhere.” – Hank Sikora

“I’m guessing it may be the transformers in a nearby power substation or motors running in a factory. Low frequencies travel farther especially thru the ground as opposed to the air.” – David Massey

“I have heard the Hum, and know what it is. It is the sound of wings drying. Millions of insects hatching and drying their wings before flight. In my case, it was the Mayfly in south central U. S.” – Clayton Burger

“Heard it in Coralville Lake Reservoir, Iowa City, Iowa when under water throughout the summer of 2011. Myself, including my sons, and some of their friends heard a unique hum. We heard a pulsating hum while swimming under water. We were curious and continued to go under water and show all we could that would be willing to hear it under water. Was a “whah whah whah” and we could only describe it at the time as what it would sound like is: gigantic bullfrogs. We all insisted it was not other boats, and it was not the dam at the other end of the lake. We heard sound throughout the lake no martter where we were, and others on the boat laughed. At the time, we joked it was going on as if from in the center of earth. To hear it tonight that others have heard it around the world is chillng.” – Brent Kottemann

“It is the US Navy project called HAARP.” – Tessie Foley

“I have often heard a hum in the night — ONLY in the night, over the past few years. I have been unable to determine the source of the hum. It is a VERY ANNOYING hum that creates a bad headache, because it never seems to stop. When it is happening, I have trouble concentrating and can’t go to sleep. … The only excuse I’ve been able to come up with is maybe the vent on top of the house is turning and making the hum. I hear it more in the winter than the summer, but I think that’s because the winter is more silent at night and sound travels better, since there are no leaves on the trees to muffle the sound.” – Renee La Viness

“I have been the lucky recipient of my own insomnia infecting noise at my home in Eudora, Kansas. It all started around Easter of this year. This is a rural area, so weird noises and things going bump in the night stand out. After about a week I went investigating and pinpointed to a newly installed oil pump about a mile from my house…When the pumps kick on, they vibrate sending sound waves out until they reach a conduit like home’s foundation.” – Michael Klaasmeyer

“I have heard this hum for about 5 years. I have heard it in Boulder, Colorado, Moab, Utah, Aptos, California. I notice that it happens when the weather is colder than 60 degrees or so. Especially if there is a sudden drop. It is definitely mechanical/man made. … I believe that it is an accumulation of machines that create a low frequency wave pattern that is heard by people with acute hearing. … For those of us that hear it, it is a torture. (I can verify, the local Walmart fixed a unit on their roof after I drove around and thought that the noise was coming from their roof units). I am just glad that I am not alone.” – Colleen Callin

“I live in Oakland CA & since last month my neighborhood listserve has been replete with complaints about a humming noise at night! … sounds like the hum of a lawnmower or some other type of machinery. It’s usually audible by 9pm and it continues throughout the night until the morning. It is a constant ort of hum that seems to change pitch every once in a while.” – Amy Dawson

“I live on Kauai, Hawaii. … My theory: We have a small naval base here, lots of radar, perhaps the hum is linked to this.” – S.C. Aquino

“I used to hear a pulsating hum at night, but not every night. I finally realized that it was the train down in the valley about 5 miles from where I lived. I think atmospheric conditions have to be just right in order to hear it, and sound seems to carry more at night. I now live one block from the railroad track, where the freight trains will often sit and idle, so I can now hear ‘the hum’ up close. It does make a pulsating sound and it seems to be louder at night, and especially during very cold weather.” – Julie Peterson

“Here in NE Ohio, my mother and I both hear a ‘hum’ at night. We live about 5 miles apart. I went outside to investigate and thought I’d discovered the cause: the cooling fans in the big electronic AT&T boxes just a few yards away from my condo. I guess as usage goes up, the fans come on.” – Carol Kotun

“Sounds like lonely bullfrogs to me.” – Bill Pierpoint

Submit your theory for what is causing “the hum” HERE.