6 feet under the landing mat, and he knew it. ''That's it, I'm done,'' Paul Hamm told himself Wednesday night, after he had tumbled from first to 12th with a slip of a foot on the vault.

And then, the best men's gymnast the United States has ever produced picked himself off the floor and pulled off the most astounding comeback in the history of the sport, nailing his final two sets to win the all-around gold by the slimmest margin (.012) in Olympic annals.

''Unbelievable,'' the 21-year-old native of Waukesha, Wis., declared, after he had outpointed South Korea's Kim Dae Eun with a spectacular high-bar finale to become the first U.S. champion in the event's 104 years at the Games and first medalist since Peter Vidmar in 1984. ''I didn't think it was possible once I was in 12th place.''

Neither did anyone with a calculator. Not only did Hamm have to perform all but flawlessly on the parallel and high bars, his rivals also had to crack.

After three of the six rotations, the Yanks were looking good for both gold and bronze, with Hamm and Olympic rookie Brett McClure sitting first and third.

Then came the vault, which felt good for Hamm going up, short coming down and dreadful upon landing.

''Once I landed, I couldn't stop the momentum off to the side,'' said Hamm, who had to be propped up by outstretched arms to avoid falling into the judges.

After the predictable 9.137 went up, Hamm sat on the sideline, staring into space in disbelief and exasperation. ''I probably just cost myself any medal,'' he reckoned.

''Paul was still composed,'' said coach Miles Avery. ''He knew he'd dug a hole for himself, but he knew he could still get a medal. And he did not want to give up that medal.''

What Hamm needed was a few slippery hands and wobbly knees around him. One of his top rivals, Japan's Hiroyuki Tomita, had blown up on the first event, tumbling way out of bounds on the floor.

Hamm led off the parallel bars with a marvelous outing, sticking his landing as if he were wearing flypaper slippers for a 9.837, the highest score on the event all week. ''Then a few more things went my way,'' he said.

China's Yang Wei lost control on the high bar, was left hanging by one hand, and took a devastating 8.987.

Then Romania's Ioan Suciu took a 9.312 and teammate Marian Dragulescu a 9.437 on P-bars and Japan's Isao Yoneda was saddled with a 9.025 on high bar.

Finally, it came down to Hamm and Kim, who had just come off the floor pumping his fists after getting a 9.650. ''On the last apparatus, I believed that I would achieve the gold medal,'' said Kim, who had been only 11th in qualifying.

Even to tie Kim for the gold, Hamm needed a 9.825, more than three-tenths higher than he had gotten in Monday's team final, when his hand slipped and he finished 17th on the apparatus. But it wasn't anything Hamm had not done in practice. All he needed was to do something no U.S. male in more than a century had done.

Three consecutive release moves, letting go of the bar in mid-air then grasping it again. Then a dismount that had to be as sure as Neil Armstrong's landing on the moon.

Once Hamm did it, and saw the 9.837 go up on the board, he knew he had the silver. ''You're Olympic champion,'' Avery shouted to him. ''What? No!'' an astonished Hamm replied.

Three years ago in Belgium, Hamm had the world gold medal in his grasp until he smashed his face on the high bar and ended up seventh.

Wednesday night, the gold came back around for him in the place where the Games began. ''The athletes talk about their dreams,'' USA Gymnastics president Bob Colarossi was saying as the laurel wreath was being placed on Hamm's head. ''This is the stuff dreams are made of.''