Act One was self-explanatory. Hibernating cicada nymphs emerged from the ground and crawled around.

Act Two also was straightforward. They molted and left behind a diaphanous, buggy husk. They began to fly.

Act Three, the Washington region has learned over the past week or so, is more mysterious and complex, something you almost have to experience to understand. Words seem inadequate to describe that vaguely menacing hum-whistle that seems to be everywhere but emanates from no single place in particular.

Hollywood imagery helps.

"It feels like an alien spaceship coming in," said Arlington resident Gene Miller, 66.

"You ever watch that old 'Star Trek' episode where they leave their phasers on and try to melt something?" asked George Fox of Alexandria. "That's what it sounds like to me."

We've entered the peak of the Brood X emergence -- a roughly three-week period when millions of male insects are doing what evolution designed them to do, which is mate before they die. That means the males are singing, often in unison. The collective mating call produces what many described in a sort of shorthand as, "you know, the UFO sound."

Male cicadas gather in densely packed "choruses," projecting what entomologist David Marshall said is "among the loudest sounds in nature." The females are silent.

"If those males don't mate, they've utterly failed," said Marshall, of the University of Connecticut. "So everything they're doing is centered around that sound."

The noise probably will last until the third week of June, when most of the cicadas probably will be dead, said Gaye Williams, a Maryland Department of Agriculture entomologist. Predicting precisely when the emergence will end is difficult, she said, because it depends on many variables -- temperature, moisture, humidity.

They sing until the bitter end, she said.

"When they're dying, it's the last thing that goes," she said. "At the very end, it almost sounds like a heart monitor. It's very sad."

The singing is mostly a daytime phenomenon. At night, cicadas for the most part go quiet because cooler temperatures and darkness calm them down. Conversely, direct sunlight and heat amplify the racket.

"That's the main thing the males are doing out there," Marshall said. "They're doing some feeding, but other than that, they're calling to convince the females."

The sound is produced by membranous panels beneath the cicada's wings. The panels, called tymbals, vibrate rapidly. The cicada's body, mostly hollow, serves as an amplifier. Even the bug's eardrum gets in on the noise making, vibrating along with the tympanum to produce an even louder noise.

Each species has a signature song. The noise we hear in the Washington area is produced by three different species, all part of the emergence.

Although it can be hard to tell when the cicada chorus is fully amped up, there are dozens of distinct sounds. Some cicadas prefer to whir. Some whistle. Others click.

When a male cicada's song leads to success, specific sounds lead up to the procreative act, and then others are emitted during the denouement, Marshall said.

"It's a very fast ticking," he said.

Cicadas also emit a distress signal when they are threatened or trapped, a sound that is louder, less musical and more rattlelike than the mating call.

This seems to unsettle listeners more.

Staci Largen, 27, said she was awakened one recent morning in her fifth-floor Arlington apartment by what she initially thought was a person screaming. The noise came through her closed window. She later bought earplugs so she could sleep.

"It sounds like somebody getting ax-murdered," Largen said. Others likened it to the thrushing squawk in Alfred Hitchcock's horror classic "The Birds."

Gene Miller, a retired American Airlines freight handler, said it provides a "pleasant" soundtrack to his daily routine. He was scribbling in a book of logic problems -- his late-morning diversion of choice -- recently in Fort Ward Park in Alexandria.

The UFOs were descending.

"I think it's fantastic. I just like it, okay?" said Miller, sipping a Diet Coke at his favorite picnic table. "When they go away, I'm going to miss them."

The chorus can reach about 90 decibels, experts said. That's roughly the equivalent of listening to a lawnmower from about 10 feet away.

The noise isn't likely to cause hearing damage, said Brad A. Stach, president of the American Academy of Audiology in Reston.

Eighty decibels is generally considered the threshold for hearing-damaging noise, Stach said. But it takes prolonged exposure to noise above 80 decibels to begin causing problems.

"The likelihood that [cicadas] are going to cause you any permanent problems is small," Stach said.

It is the haunting vibrato of the cicada choruses, less than the volume, that seems to inspire artistic interpretation. Tamara Smyth, a lecturer at Stanford University, designed the tymbalimba, an elaborate metal instrument which is played with the fingers, like a piano. It interacts with a computer to produce music.

George Fox, an Alexandria Web designer and amateur composer, has written a song called "Brood X" (http://www.f2sys.net/brood-x/), which he produced with synthesizers. Melded into the throbbing electronic baseline are recordings of real cicada noise.

"The rhythm track, what would be normally played on a cymbal or a high hat, are actually cicada," Fox said.

Fox also wrote a song for the cicadas during their last emergence, 17 years ago. He tried to reproduce it by turning a bicycle upside down, spinning the wheel and clicking drumsticks across the wheel and spokes.

"It was really experimental," Fox said. "I don't know if it was all that listenable."

It's a tough noise to describe, much less duplicate. When words fail, even such experts as the University of Connecticut's Marshall resort to mimicry.

"Wheeeeeeeeeeroo, wheeeeeeeeeroooo," the entomologist said. "Wheeeeeeeeeroo."

A cicada exoskeleton, framed by a U.S. flag in a Pennsylvania yard, hangs from a branch. The Washington region is at the peak of cicada emergence.