Around 5 a.m. Monday, a loud sound moved through parts of Anchorage.

It came in bursts, getting louder, then evaporating. It was heard from Midtown to West Anchorage, from Spenard to the Old Seward Highway. Some heard it once, others repeatedly for hours. People miles apart reported the tone abruptly waking them up.

“It’s almost like a foghorn, and screeching metal on metal,” said Jamie James, who heard the sound as she was getting ready for work in her home near Lake Spenard.

“Every time I tell somebody about it, I say it sounds like a submarine scraping the bottom of a pool or something,” said Melissa Thompson, who went outside to record the sound.

“I would say it sounded like underwater moaning,” said Stephanie Quinn-Davidson, who lives near Ted Stevens Anchorage International Airport and has heard the sound multiple times over the years.

Some Anchorage residents went on social media to joke about the sound’s origin, saying it was someone’s hungry pet dinosaur or that a “big backhoe was giving birth.”

Like others, James knew the sound, but the origin was mysterious. It’s eerie, described by some on social media as symphonic. It’s also sporadic, ramping up for about 20 seconds and then disappearing.

The sound has been heard around Anchorage for years, but there was a sharp uptick of reports on Saturday and Monday. James posted about it on the social media website Nextdoor, and by Tuesday there were 88 comments. People reported reaching out to their community council and Anchorage Assembly members to complain.

For every explanation floated on social media, there’s a counterargument. Reports of the noise come in the summer, when plows aren’t operating. Some reports are miles from the airport, casting doubt on the claim that graders breaking up ice could make such a loud noise.

Anchorage airport spokesman Eland Conway listened to one video and suggested it was the backup beeper for heavy equipment. But then, Conway said he checked with the maintenance department and they were not doing work that would cause that kind of sound Saturday morning, when Thompson recorded her video.

Thompson first heard the noise four years ago.

“If there was a sound that could just change your matter or something, I don’t know,” she said. "It’s a sound that’s so internally loud. You feel the sound.”

Conway said Monday he wasn’t aware of any noise complaints. Conway doubted the snow-removal work at the airport would cause such a sound.

“It would be hard to believe that it would be loud enough to be heard all the way over by Lake Otis,” he said.

Alan Czajkowski, Anchorage’s director of maintenance and operations, said whatever it is, it’s not coming from city workers or equipment.

“There’s nothing we would be doing that’s that loud," he said.

Alaska Railroad spokesman Tim Sullivan said if there was an unusual sound coming from trains, the public would promptly let him know.

“Nothing here that I am aware of, other than our usual industrial sounds,” he said.

One of the most popular theories has an unusual name: “trumpets in the sky.” A noise similar to what’s being heard around Anchorage has been recorded around the world. People subscribing to the theory tend to believe the phenomenon happening in Anchorage is related to the noises elsewhere, but there isn’t a consensus on what, exactly, that phenomenon is.

Anchorage resident Jennifer Kettleson created an Anchorage-specific “trumpets in the sky” Facebook group in 2016 to track local reports. Reports of the sound were posted on the page Saturday, Monday and Wednesday mornings.

Kettleson said in a Facebook message that she created the group because she thought it would be fun for people to pool their information and try to find out where the sounds are coming from.

Four years later, the group still hasn’t figured it out. However, the recent reports caused a surge of participation in the Facebook group. Followers have trickled in over the years, Kettleson said, but in the last week she approved 126 requests. Most came Monday and Tuesday. The group now has 649 members.

She personally believes the sound is coming from the Earth and is connected to earthquakes.

News outlets and message boards around the world have documented the reports. YouTube has ample audio posts.

Some articles reference NASA saying the sounds are naturally occurring “background” noise from the planet.

Rob Gutro, a NASA spokesman, said Tuesday that he was unable to find a scientist at the administration to address the phenomena. But he pointed to the late U.S. Geological Survey scientist David Hill, who published a paper in 2011 about reports of “mystery boom” sounds. Hill suspected there were several culprits, from military exercises to earthquakes to “explosive offshore methane bursts.”

Thompson first heard the sound in Anchorage about four years ago, and it scared her. She ran inside. Most recently, she heard it at about 5 a.m. Saturday, from her house at 76th Avenue and the Old Seward Highway.

This time, she ran outside to record it. It’s still “creepy,” she said. She posted her video to Kettleson’s Facebook page on Saturday, and the post had been shared more than 200 times by Thursday morning.

Thompson found the group after searching the internet for “weird sounds,” and that’s where she came across the “trumpets in the sky” theory. In the videos, she heard the same sound she hears in her neighborhood from time to time.

“I think it is all the same, I just don’t know what the cause of it is,” Thompson said. "It’s definitely a phenomenon of some kind.”

Some who heard the noise looked to government officials. On Monday morning, a constituent emailed Assemblywoman Austin Quinn-Davidson to complain about the sound. The person wanted to report the noise as a public nuisance.

Quinn-Davidson didn’t have answers. But she, too, had heard the noise.

“Literally, I wear earplugs to sleep at night, and it woke me up,” Quinn-Davidson said. "It’s so loud. It sounds somewhat mechanical.”

Despite being heard for miles, no one seems to know the origin. James hoped her Nextdoor post would lead to answers. But just as everyone has a different description for the sound, they also have a different explanation.