A blast wave at 3:35 am rippled through the woods around Lonely Cottage Road in Upper Black Eddy. Frogs stopped croaking. Crickets ceased chirping. And Nick Zangli loaded a gun to defend himself from whatever it could be.

He found an eerie quiet following what he would later call "a very large explosion" in that part of rural Pennsylvania.

The next day, someone found a four- or five-foot-wide crater, about a foot deep, alongside a back road shaded by dense trees. Debris was scattered along the pavement, Zangli told ABC affiliate WPVI.

"We shoot guns and pop fireworks up here for fun, but this was above and beyond," Zangli said, perplexed, outside his Upper Black Eddy home.

Hes not the only one baffled - and that was hardly the only mystery blast.

There have been at least 19 others in Upper Bucks and Lehigh counties since April 2, and state troopers and ATF investigators are at a loss to publicly explain what is going on.

Details from Pennsylvania State Police have been slim.

Master Trooper Marc Allen, a spokesman for the force, told The Washington Post that authorities do not want to tip off potential suspects about how much investigators already know about the string of explosions throughout the county.

Here is what is publicly known.

All of the explosions occurred between 1 am and 4:30 am over the past two months, and no one has been injured by any of the blasts, according to a statement provided by the state police.

The explosions are intentional and not anything caused by geologic, construction or any other means, Allen said.

Thats about it.

Charlene Hennessy, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said Wednesday that the agency was involved in the investigation in a limited capacity but did not offer details about the federal response, which includes agents from the FBI.

Yet Hennessy previously speculated about the cause, telling Lehigh Valley Live that the blasts could have been caused by large amounts of explosive firearm targets called Tannerite - which are sold legally and not regulated by ATF.

Tannerite-brand targets are sold as separate components that, when combined, can produce a small explosion.

Blowing up large amounts of Tannerite is its own YouTube genre. In one video, a group of enthusiasts gathered 164 pounds of Tannerite in a plastic bin and stuck it inside an abandoned barn, as explained by a man wearing a Richard Nixon mask, sunglasses and ear protection.

The barn is obliterated in the video.

"I would not be surprised if thats what it turns out to be," Hennessy told local media, referring to Tannerite.

Still, the paucity of information has left locals to speculate about the nighttime blasts, with guesses including satellites falling from the sky and illegal drilling.

Sue Crompton said an April 30 explosion shook the earth and rattled the windows in a trailer home shared with her mother.

"It practically knocked me out of bed," Crompton told Philly.com. "Your mind goes to meteorites or people making bombs. I just don want anyone to get hurt."

Crompton knows explosions: She told the Inquirer she previously worked at a ski resort in Colorado and sometimes watched controlled demolitions to blunt dangers from sudden avalanches.

That April explosion told her everything she needed to know - it wasn a fire, even though two weeks earlier, two blasts about five minutes apart led her neighbors to suggest a fire at a nearby barn must have been the source.

Zangli, who was jolted in Upper Black Eddy on Mothers Day, does not believe the explosions are nefarious.

But hes worried about what could result if they continue.

"Probably kids that come up with a formula to blow stuff up, and you know its cool to watch something blow up," he told the local ABC outlet, "but you could kill somebody in the process."