For more than three days, people in Clintonville, Wis., have woken up from sleep to a noise that some described as sounding like an explosion or distant fireworks.
What is it? No one knows. Geologists have ruled out earthquakes or explosions in old mines. City officials checked gas and sewer lines. The local military says they're not running any drills.
"In summary, it's crazy," Brian Niznansky, a meteorologist for Wisconsin's NBC26 station, said. "The craziest part is that no one has an idea, from the local officials to scientists, whether it's natural, manmade, a hoax, no one has any sort of idea what is going on. You can't rule out weather because we're in [a] historic heat wave."
Warm weather could potentially be to blame.
Lisa Kuss, spokesperson for the city, said the same, at a town meeting called on Tuesday to talk to residents about the disturbing sounds.
"It is a strong possibility that some natural phenomenon is occurring under the ground. We have discussed that both our strange winter weather and current very warm spring like weather, which is early, is extreme at best and somehow could be impacting our soils below the surface," Kuss said.
One theory about the noise is that the sound comes from the ground settling after ice in the ground quickly melted.
"If we've had a cold winter, and you get a good frost layer, 4 or 5 feet, then you get a huge warm up, you might get some cracking of the ground but our winter was very mild," Niznansky said.
"Cryogenic" noises, sounds related to freezing of the ground, are unlikely. Wisconsin had a warm winter. There were not many days with temperatures below freezing so that a thick frost layer could form. Temperatures were around 7 degrees above normal in January, almost 8 degrees above normal in February and 17 degrees above normal so far in March. The winter season's average temperature was 6F above normal.
"Over the last four days, Wisconsin had a dramatic increase in temperatures, from 20 degrees above normal to 35 degrees above normal," Margusity said. "Since Friday, the temperature has really increased rapidly. At the beginning of March, the temperatures were in the 30s. Wednesday was 81 degrees."
Margusity compared the ground warping to a bridge expanding and contracting in the cold.
"Between the wind, sunshine and humidity, you're warming the ground very rapidly, like a pan on the stove that warps because it gets hot so fast," AccuWeather meteorologist Henry Margusity said.
As July draws to a close, a storm system swinging up from the Deep South will bring downpours to the northeastern U.S. and break the back of an extended heat wave.
Repeating and slow-moving storms will raise the risk of flash flooding and damaging winds over northern and central High Plains into Thursday night.
The F1 season continues this weekend with the German Grand Prix in Hockenheim with disruptive showers and thunderstorms in the forecast.
Highs will run between 10 and 15 degrees Fahrenheit above average across much of the interior western United States into the upcoming weekend.
The heat felt across the United Kingdom during the middle of July has faded and is not expected to return through at least the first week of August.
Repeating downpours will raise the risk for flash flooding along the Gulf coast and lower Mississippi Valley through the middle days of the week.
Marquette, Il (1988)
99 degrees for a date record.
Hurricane Bertha formed 450 miles east of Jacksonville, FL. Maximum sustained winds of 75 mph with gusts to 90 mph.
Western Pacific (1990)
Typhoon Steve east of Iwo Jimo. Peak winds of 125 mph sustained gusts to 155 mph.