After confounding visitors at Magnuson Park on Tuesday, by Wednesday it had vanished.
Now it has reappeared in another Seattle park, on an island in Green Lake.
Nobody has taken responsibility for the disappearing and reappearing monolith.
"It's pretty amazing," said city spokesman Dave Hughbanks.
Dozens of people went to the site Wednesday hoping to see the monolith, which is made of steel and measured about 9 feet tall, 4 feet wide and 1 foot thick.
Its removal from Magnuson Park does clear up a few questions, Hughbanks said.
"We found out they poured a cement base for it to sit on," he said.
After the cement dried, which would have taken some time, sod removed for the pouring was neatly replaced against the monolith itself.
"There were no tire tracks to the site" and the method of its delivery to wind-swept Kite Hill, overlooking Lake Washington, is still not known, Hughbanks said.
"Yesterday it looked like it had come out of the sky. It could have," he said with a twinkle, clearly enjoying this second phase of what apparently qualifies as the city's latest guerrilla art escapade.
"It was beautifully welded," Hughbanks noted.
No one has taken credit or responsibility for the monolith. A call to a local art foundry was not returned.
Originators of a possibly connected Web site www.supportthemonolith.org could not be traced. The site, now down, featured a doctored copy of the famed WWII photograph of soldiers raising the flag at Iwo Jima. In this version, they were placing a monolith.
Recalling the movie, in which monoliths from outer space somehow trigger evolutionary advances, the site's authors noted that aliens have made no such deliveries lately.
"Rather than let these facts discourage us," the Web site says, "some people have taken matters into their own hands."
Skid marks near the monolith's brief location at Magnuson Park suggest the hollow piece "it rang" when thumped, Hughbanks said was dragged for a distance and then possibly placed on dollies or lifted and carried.
"I'd suspect it'd take six or eight men with pretty strong shoulders to sling it," he said.
There's a boat launch about a half mile away. Gates bar vehicles from park grounds overnight.
"They could have loaded it on a boat. Or a silent helicopter or a space ship," Hughbanks suggested. "It may have gone to another planet. Who knows?"
There were no sound effects no tape recordings of the 2001 theme, also known as Strauss's "Thus Spake Zarathustra."
"Nor were there any monkeys running around that we could identify," quipped Hughbanks.
One scene in the movie suggests the monolith's appearnce prompted the first use of tools by humankind's hairy precursors.
Other recent guerrilla art endeavors include the installation of a 700-pound ball and chain on the Seattle Art Museum's Hammering Man on Labor Day 1993 by a group calling itself "Subculture Joe and the Fabricators of the Attachment."
"Subculture Joe" got a lot more notice in July 1996 when he abandoned his truck and its cargo an 1,800-pound metal heart at Westlake Park downtown. The nearby area was evacuated at great expense and inconvenience because police worried that the truck might carry a bomb. It was just a heart.
Just before Christmas 1996, pranksters decorated the Hammering Man with a Santa hat that was hastily removed by museum officials.
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