Oakland - Did it come from outer space? Was it a transport vehicle for illegal aliens of the extraterrestrial kind? The tail of a comet grabbed by gravity?

Jokes were flying Saturday morning after a block of solid ice, measuring more than two feet on a side, crashed to earth with a tremendous bang, digging a three-foot hole in the grass at Bushrod Park, 5800 Shattuck Ave.


But when the laughing stopped, an expert theorized it probably fell from the wheel well of a plane landing at Oakland or San Francisco International airports. It also could be an unexplained "ice fall," one expert said. Big balls of ice sometimes fall from the sky without any real explanation.

Wherever it came from, it's arrival was heart-stopping and dramatic.

Brooks and Judith Mencher said they were standing on their back porch on 59th Street near the park when they heard a sound like a very loud rocket. "It kind of went 'whoosh!'" Brooks Mencher said.

The impact hole looked like it was created by a hand grenade, said Oakland Police Sgt. Ron Lighten. "It knocked turf 20 feet away."

Lt. Charles Glass of the Oakland Fire Hazardous Materials Team said the ice was pure water. " It didn't come from a toilet on a plane or anything like that."

Glass said the ice that firefighters pulled from the hole was about the size of the hole, three by three feet and two and a half feet deep. It also broke a ceramic irrigation pipe at the bottom of the hole.

At Oakland International, a spokeswoman said she had no idea if a plane might have been going overhead at that moment. "We'll have to wait until Monday, when that information is available," she said.

Web sites that track commercial flights show a spike in arrivals at Oakland International at about the time, 10:05 a.m., that the ice ball put a hole in Bushrod Park. Two runway approaches for San Francisco International also go almost directly overhead.

An operations manager at the Federal Aviation Administration in Los Angeles said he had never heard of such a thing.

"I've been here 15 years and what usually falls from planes is 'blue ice,' that's methylene glycol. They put it in airplane toilets. Sometimes there are leaks and it falls out," he said.

But Tony Hirsch, a Columbus, Ohio-based aviation expert, said ice falls of pure water are not uncommon: "Ice builds up on airplanes and falls off as they prepare to land."

But Hirsch said the airplane "would have to descend through what we call visible moisture, rain or clouds, for ice to build up." The skies were partly cloudy Saturday morning.

Hirsch said a large chunk of ice could build up on the vertical stabilizer or in a wheel well: "When they lower their landing gear, it falls off."

As bad as Bay Area weather has been, the National Weather Service said none of the storms has been violent enough to hatch a gigantic hailstone on its own. "There's nothing meteorological that would create a piece that big falling into Oakland," said weather service forecaster Diana Henderson.

Electrical engineer Richard Spalding thinks there must be another explanation. He tracks meteors as part of work on satellite instruments and became interested in balls of ice that smash down from time to time.

"Ice falls do happen fairly frequently," Spalding said. "Just about every year, there's a news item somewhere. I think it came from a natural process that we're ignorant of, where it can form at altitude and fall as a chunk," he said.

There's at least one recorded incident of ice from a plane hitting a house. It happened in 2004 in Kent, near Seattle. Homeowner Troy Halte said Saturday a chunk of ice made a basketball size hole in his roof and the ice landed in softball-size pieces on this daughter's bed.

"Fortunately, nobody was home," he said.