OAKLAND — Ice falling from the sky might seem unusual, but some Spanish and American scientists say it is becoming a frequent occurrence throughout the world.

Like the estimated 200-plus-pound chunk that fell Saturday on Bushrod Park, clear ice from the sky has been reported around the world. Big and small ice-falls have happened in China, Spain, Italy, Czech Republic, Scotland, Hungary, England, India and more than half of the United States — often in summer and some recorded before aircraft were invented, scientists say.

And in each case, no one knows why.

"None of us have been able to come up with a process to determine how it is happening," said David Travis, a climatologist at the University of Wisconsin-Whitewater. "We're really baffled as to what is going on here."

Travis and Madrid-based scientist Jesus Martinez-Frias have studied the phenomenon since at least 2000, when they began scouring the Internet for news stories about the incidents. They found 37 instances in which a large chunk of ice fell from clear skies. Each year, they find more reports of it happening.

The Oakland ice cube was clear and free of debris, ruling out any chance it came from an airplane bathroom, the experts said. But its large size makes it hard to believe the ice is a product of nature.

So conflicting theories abound.

Martinez-Frias speculates it is a natural phenomenon caused by global warming. According to his studies, every time such an incident occurs, it is precipitated by an unusual atmosphere in which higher altitudes are turbulent and cold. The cold helps create the ice. The turbulence helps keep it together in the sky.

As global warming continues to heat the earth, his theory goes, upper atmospheric temperatures become cooler, opening more opportunities for the ice to form. Charles Knight, a leading hail expert at the University Corporation for Atmospheric Research, told an interviewer in 2002 the "meteorological explanations just don't make sense to me" for creating giant ice balls way up in the dry stratosphere.

"I don't like to claim that anything is absolutely impossible, but this comes awfully close," Knight told Science magazine.

While Travis said he understands the global warming theory, he thinks gravity is too strong to keep that big a piece of ice in the sky.

Instead, Travis believes the ice forms on the underbelly of an airplane, maybe near the landing gear. Once the plane prepares to land, the landing gear opens, dislodging the ice.

"I know the Federal Aviation Administration will deny this has anything to do with airplanes," Travis said. "But it is very difficult to imagine any other explanation."

Representatives for Oakland and San Francisco International airports referred calls about Saturday's incident to the FAA.

In the late 1990s, when a huge, 400-pound chunk crashed through the roof of a Mercedes-Benz factory in Southern Brazil, U.S. defense scientists analyzed it for signs of cosmic origin. The water's isotopic signature indicated the ice ball was terrestrial, with the water coming from temperate latitudes. Beyond that, tests were inconclusive.

In January 2000, an ice ball dropped from a bright, blue sky through a car windshield in Spain, soon followed by several others in nearby towns.

Spanish scientists performed the most rigorous analyses yet on the ice chunks, which weighed about a kilogram, and dubbed them "megacryometeors." No planes were found overhead in two of the bombardments.

The Spanish team theorized the ice formed something like giant hail, but instead of bouncing around at the cold tops of thunderheads, these would drop from much higher, out in the frigid stratosphere — when it is especially wet and depleted in ozone.

That is how the Spanish team conjured large pieces of ice without rain, sleet or snow.

Oakland was wet and so was the air high up, but tropospheric conditions overhead Saturday were "nothing extraordinary," said Warren Blier, science officer for the National Weather Service's San Francisco regional office. 

It is hard for him and others to imagine the Spanish recipe for mega-hail working at all, much less for the 200-plus-pound slab of ice that put a dent in Oakland.

"It's very, very, very unlikely. It's hard for me to conceive this would occur. There are much higher probability explanations, and aircraft are at the top of the list," he said.

Staff writer Ian Hoffman contributed to this report.