LATE on the evening of May 28, 1993, something shattered the calm of the Australian outback and radiated shock waves outward across hundreds of miles of scrub and desert. Around the same time, truck drivers crossing the region and gold prospectors camping nearby saw the dark sky illuminated by bright flashes, and they and other people heard the distant rumble of loud explosions.
The mysterious event might have been lost to history except for the interest of Government investigators in Australia and the United States who eventually came to wonder if the upheaval was the work of the Japanese doomsday cult accused of the poison-gas attack on Tokyo subways in 1995 that killed 12 people and hurt thousands.
The fear was that the terrorists had acquired nuclear arms or other weapons of mass destruction and had been testing them that night in the Australian wilds.
The hope was that the upheaval was an earthquake, a mining explosion or even a meteorite strike from space, any natural event.
The evidence was ominous. Investigators discovered that the cult, Aum Shinrikyo, had tried to buy Russian nuclear warheads and had set up an advanced laboratory on a 500,000-acre ranch in Australia near the puzzling upheaval. At the ranch, investigators found that the sect had been mining uranium, a main material for making atomic bombs.
The clues were judged worrisome enough to set in motion a wide scientific investigation that is still going on today.
''Many experts had dismissed the possibility of nukes'' in the hands of terrorists before the emergence of Aum Shinrikyo, Dr. Gregory van der Vink, head of the science investigation, said in an interview. ''But the group was into biological and chemical weapons and was attempting to acquire nuclear ones. I'm still amazed.''
At the request of Senate investigators, the science inquiry was led by the Incorporated Research Institutions for Seismology, or IRIS, where Dr. van der Vink is director of planning. The IRIS consortium is based in Arlington, Va., and financed by the National Science Foundation, a Federal agency. It has more than 80 member institutions at universities in the United States. It also has more than 100 seismometers on all continents, the largest global network of these devices, which pick up faint vibrations traveling through the earth's rocky interior.
One aim of scientists using the IRIS network is to try to distinguish natural disturbances from nuclear explosions, a goal that recently took on new significance as the United Nations called for an end to all explosive testing of nuclear weapons around the globe. Much research, including that by IRIS members, now aims at monitoring such a ban.
Pinning down the nature of the outback incident was seen as an important test of the emerging skills.
The doomsday cult that caused the worry first caught world attention after the Tokyo subway attack of March 20, 1995, which involved the deadly nerve gas sarin, and was the world's first large chemical strike by terrorists. The cult was quickly implicated and is now charged in Japan with planning a virtual civil war meant to be carried out with some of the world's deadliest weapons.
After the Tokyo attack, government investigators around the world raced to learn more about the shadowy group. Aum Shinrikyo, or Supreme Truth, turned out to have accumulated some $1 billion and to have won more than 50,000 converts in at least six countries.
In the United States, the permanent subcommittee on investigations of the Senate Governmental Affairs Committee, at the urging of Senator Sam Nunn, mounted a major investigation of the cult.
Senate investigators found that the group had thousands of disciples in Russia and had bought guns, a military helicopter and other weapons there. The group's construction minister, Kiyohide Hayakawa, the reputed mastermind of the cult's efforts to arm itself, went to Russia 21 times from 1992 to 1995, visiting there a total of 180 days.
Senate investigators say the cult recruited at least two nuclear scientists in Russia.
Notebooks later seized from Mr. Hayakawa show he wanted to buy the ultimate munition there. In one entry, he asked, ''How much is a nuclear warhead?'' and listed several prices.
In Australia, the activities were just as troubling. Cult members arrived in April 1993, a little more than month before the mystery blast. Mr. Hayakawa, apparently fresh from visits to Russia, was among the initial party. After visiting several remote sites, the group bought a 500,000-acre sheep farm in Banjawam, Australia, about 400 miles northeast of Perth. The site has a known uranium deposit.
The cult eventually brought in chemicals, gas masks and respirators, and picks, shovels, mining equipment and a mechanical ditch digger. It also set up a laboratory stocked with computers, glass tubing, glass evaporators, beakers, Bunsen burners, mixing bowls and a rock-crushing machine.
Documents seized from Mr. Hayakawa include some 10 pages written during his visit to Australia in April and May 1993 that refer to the whereabouts of Australian properties rich in uranium, including one reference praising the high quality of the ore.