CHICAGO -- For everyone except the suburbanites whose homes and cars have been hit by blue-green goo, the strange outbreak of droppings in the Chicago area over the past three weeks has been the subject of considerable mirth.
But the bizarre phenomenon is much more mystery than comedy.
The victims are convinced that they have been strafed by the contents of airliner lavatory holding tanks.
Officials at first suspected that the droppings -- reported in patches in a wide area from Lake Michigan west to Warrenville, Ill., from the Wisconsin border south to Flossmoor, Ill. -- came from flocks of geese or ducks.
But recent developments suggest that something more is at work.
First, a lab analysis found that a sample collected in Lincolnwood contained seeds, plant material and human proteins from waste matter.
Second, an airline maintenance veteran inspected the stuff that hit his new Ford Explorer outside his Palatine, Ill., home, and he is convinced that it's something he's seen many times before on the job: waste from a plane's lavatory.
Henry Little, 46, a maintenance crew chief for American Airlines, also has a theory about how the discharges could be happening: A plane missing its doughnut plug and drain cap is flying Chicago-area skies.
Federal Aviation Administration inspectors have perused maintenance records and are checking planes at O'Hare International and Midway airports in an intensified inspection program, said Donald Zochert, an agency spokesman.
But they've found no defective lavatory systems, he said.
If the goo is coming from passengers airliners, government and airline experts are baffled about how it is happening.
That is because the backup systems and the backup-to-backup systems that are built into the power, flight control and other components of airplanes that fly U.S. skies are present in the lavatory systems as well.
Three plugs designed to prevent unintentional discharge would have to fail or be left in the open position for solid waste to be released from a typical plane's holding tank, experts say.
The drain system can be opened and closed only on the ground.
If all the plugs were open, people aboard a plane probably would be aware that something was amiss, said a government official who asked not to be identified.
"The flight crew would know about it because they would have problems with pressurization" as pressure from the cabin escaped down the drainpipe, he said. Passengers with sensitive ears would experience discomfort, he said, and "I'll tell you one thing -- you don't want to be sitting on the toilet."
"There also would be one hell of a howl in the bathroom," he said. "In fact, the lav door probably would collapse."
Jetliners sometimes develop leaks in a line used to fill the lavatory tank with disinfectant. Under some circumstances, the liquid can trickle onto the fuselage during flight and freeze into an ice chunk that gets bigger until it breaks off and falls.
But that is a far different situation from a sudden in-flight opening of an aircraft's holding tank, experts say.
Meanwhile, speculation about the culprit or culprits continues.
The probability of multiple plug failure on the typical jet that uses O'Hare or Midway is mathematically minute, an airline executive said.
The executive said he wonders whether someone in a small airplane is intentionally dumping waste out of sheer mischief.