One generation ago, a boomerang-shaped object as bright as a city street and as big as a football field appeared in the night skies over southern New York state, drawing such mixed reactions from some 7,200 witnesses that it never turned into the cosmic public event that it really was.

Now, 25 years after the well-documented but not necessarily well-remembered UFO phenomenon put Putnam and Westchester counties on the map as paranormal hot spots, the massive case study that has resulted is elevating the Lower Hudson Valley to the status of one of the three major UFO vortexes in the world.

The other two are Stonehenge, England, and Sedona, Ariz.

In a land with a heritage that celebrates Washington Irving's iconic Headless Horseman every Halloween, the sightings are also yielding new theories about why these quiet suburbs apparently are so haunted.

The most fantastic theory is developed in four books by one high-profile UFO investigator who teaches middle-school science in White Plains, N.Y. He believes that ancient European explorers built stone chambers in nearby Putnam to mark anomalies in the Earth's magnetic field that may open windows into the fourth dimension, thereby giving the supernatural universe a portal into suburban life.

Teacher and author Phil Imbrogno was among the first on the Lower Hudson scene with national UFO experts to record eyewitness accounts in 1983. At the time, hundreds of people in Westchester and Putnam were calling police to report a huge but silent lighted ship that the witnesses were certain they had never seen before.

The Federal Aviation Administration explained the strange sights as light aircraft flying in formation - a possibility that many eyewitnesses said they rejected when they spoke to Imbrogno and the other investigators.

The best pilots invariably break formation, and their engines are always audible from the ground, said Lt. Kevin Soravilla of the Yorktown police. He was a patrolman on duty the night of March 24, 1983, when he twice saw a silent and massive delta-shaped lighted ship.

The government explanation of conventional aircraft was perhaps to be expected. The Air Force studied 12,600 UFO reports from 1947 until 1969 and then stopped, in part because it said it was a waste of taxpayer money and in part because the sightings did not represent new technology or a national security threat.

As a result, the government turned all investigations over to private groups. And the Lower Hudson UFO legend only grew.

People came out in groups to watch the night skies for UFOs. New-age seekers toured the stone chambers in the woods of Kent for the chance to see a spook light or perhaps a druid ghost.

More than 1,500 people packed a one-day UFO conference.

Support groups started for people certain they were contacted by extraterrestrial beings, including a monthly gathering called the UFO Roundtable in Yonkers. Cable networks sent production crews to dramatize an already remarkable story with special effects.

In March, the History Channel aired a program that called the Lower Hudson sightings the biggest UFO vortex mystery of all time.

And why not?

The Lower Hudson Valley always has been a place, as Irving writes in The Legend of Sleepy Hollow, where ''stars shoot and meteors glare oftener across the valley than in any other part of the country'' and where people are ''given to all kinds of marvelous beliefs, subject to trances and visions.''

Patterson-based real estate broker Freddy Vicente found that out while driving April 27 on the Taconic State Parkway, when, he said, a circle of 12 or more clearly oversized lights glided past his windshield and out of sight into the night.

The burden of handling the UFO sightings - which came in the hundreds on the same night across town and county lines on at least three dates during the peak - fell on local police departments, whose officers sometimes also witnessed the same unidentified object as the public they were protecting.

''We are sworn to uphold the law and help citizens, but that night we didn't know what we had,'' said Soravilla of the Yorktown police. ''It was mind-boggling.''

Departments often had little recourse beyond calling airport control towers to find a conventional explanation.

The Lower Hudson UFO story remains more legend than popular history because the witness experience is so personal and varied. Some people screamed out in fear at the sight while others felt awe and even an attraction to it - especially those who sensed a wordless communication from the object not to be afraid.

''Maybe years from now the Hudson Valley UFOs will be looked at like The Legend of Sleepy Hollow, '' Imbrogno said.