The headline was a stunner: "Government source reveals signs of life on Jupiter." And this wasn't from some supermarket tabloid, either, but on the main welcome page of America Online, the world's largest commercial online service.

Monday, many of AOL's 5 million subscribers read the startling tale, and believed. How could they doubt? AOL president Ted Leonsis declared that his company had documents proving a massive government coverup of the facts.But as the day wore on, some AOL users noticed the date - April 1. They'd been the victims of a megabyte-size electronic hoax.

Said AOL spokeswoman Margaret Ryan, "The April Fool's hoax was designed to pay tribute to the history-making `War of the Worlds' broadcast in 1938." Orson Welles' radio play about a Martian invasion led to a nationwide panic, when thousands of listeners thought it was really happening.