What does the world's deadliest man do when he's trapped?

That's the question on everyone's minds now that the Taliban government that shielded Usama bin Laden has begun collapsing, the U.S. has put a $25 million bounty on his head and the terror mastermind's options are shrinking by the day.

For the first time, U.S. officials sound optimistic that they will eventually get him.

"The noose is beginning to narrow," President Bush said Monday after a Cabinet meeting.

British Special Air Service and U.S. special forces are combing the rugged Afghan landscape for the chief suspect in the Sept. 11 attacks, and they won't be alone. Washington has offered a hefty reward for locating or capturing the nationless Saudi and his top aides. On top of that, there's more reward money from the CIA.

"Our hope is that ... the dual incentive of helping to free that country from a very repressive regime and to get the foreigners and the Al Qaeda out of there, coupled with substantial monetary rewards, will incentivize ... a large number of people to begin crawling through those tunnels and caves looking for the bad folks," Rumsfeld said Monday.

Still, Rumsfeld warned the public not to expect immediate results.

"People can hide in caves for long periods," he said. "This will take time."

Bin Laden has four basic options: Flee by foot or air, hole up in an Afghan redoubt or simply die, possibly taking as many of his enemies as possible with him.

"He may try to hide from us, wait it out. Or he may decide to die a martyr and launch some attack when he's going down," said Herbert E. Meyer, vice chairman of the CIA's National Intelligence Council during the Reagan administration.

U.S. intelligence considers it most likely that bin Laden will remain in Afghanistan, rather than try to flee elsewhere. He's expected either to hide in one location, or move between several hideouts in the rugged, mountainous strip of Afghanistan running roughly from southeast of Kandahar, up to south of Kabul and Jalalabad.

In the past, bin Laden always has traveled with a small, armed security force. He's believed to use couriers to communicate because he knows the United States can eavesdrop on any phone conversation.

Among bin Laden's options:

— Sneak out of Afghanistan on foot.

Bin Laden could try to escape across the Pakistani border, perhaps to the contested Indian-Pakistani border area of Kashmir where he has supporters. But once he gets there, bin Laden has fewer of the mountain hideouts he has in Afghanistan, said Tim Brown, defense and intelligence analyst for Globalsecurity.org.

The other adjacent countries don't want him: Shiite Muslim Iran has its own fight with Sunni Muslim bin Laden. The former Soviet countries to the north are cooperating with the United States, and Secretary of State Colin Powell says China also would not take him in.

Bin Laden could disguise himself as a woman in an all-enveloping burqa, travel with just one or two people and even try to blend in with the region's thousands of refugees, Brown said. But bin Laden is well over 6 feet tall, and if anyone recognizes him, he would have little protection without armed guards.

— Sneak out of Afghanistan by air.

Bin Laden might have access to a helicopter, Rumsfeld said, and helicopters can fly low to avoid detection. But U.S. airborne-radar planes can spot even low-flying helicopters that follow the contours of hills.

Bin Laden could fly by helicopter to Pakistan, then take an airplane to Somalia or Sudan, both chaotic, lawless places where he was welcome in the past. But even those countries might reject him now out of a fear of U.S. military attacks, Powell said.

— Hide out in caves in Afghanistan.

Bin Laden spent years fighting the Soviets in Afghanistan, and spent many of his millions creating a network of caves and underground hideouts for troops and supplies out of the existing, ancient network of water trenches.

The United States is bombing any hideout it knows about, both south of Jalalabad and south of Kandahar, using "bunker-busters" that can dig under the surface and explode in a tunnel, and fuel-air explosives that can suck out a cave's oxygen.

The United States could also send commandos into caves, but that is extremely dangerous. Rumsfeld said he hoped booty-seeking Afghans would make American ground forces unnecessary.

If the job of finding bin Laden falls to the U.S. military, it will require different kinds of troops than the special operations forces now in Afghanistan, Rumsfeld said. He did not elaborate, but other officials have said an infantry unit like the Army's 10th Mountain Division might get the assignment.

If bin Laden is killed in the bombing of a cave, U.S. officials may never recover a body and never know for sure that he is dead.

— Choose to die.

In his most recent interview, bin Laden said he was "ready to die" and predicted attacks against Americans would continue even if he's gone. He might refuse to be captured alive, blow himself up and hope to take American troops with him.

"It's awfully tough to pinpoint one single person," Meyer warned, giving America a 50-50 chance of catching bin Laden.

"Even that is pretty good," he said.

The Associated Press contributed to this report.