More than 18,000 feet above the mountains on Hawai'i's biggest island, two B-2 stealth bombers drop six 2,000-pound inert bombs on a training range below.

It's a scene being repeated monthly, as the Air Force's sleek, boomerang-shaped planes regularly use Hawai'i for target practice. The aim is to make sure pilots are trained and ready to act if needed. The Missouri-based bombers have been assigned to Guam to deter North Korea and fill gaps in the regional U.S. military presence created by deployments to Iraq and Afghanistan.

"There are very few potential adversaries in the world that don't understand and respect what this bomber capability can bring," said Col. Timothy Saffold, deputy commander of the 613th Air and Space Operations Center in Hawai'i.

At a cost of about $1.2 billion each, the B-2 bomber is designed so that it doesn't show up on radar, giving it a unique ability to penetrate an enemy's defenses and go after heavily defended targets. The plane was first shown to the public in 1988 and became available for military operations in 1997.

The planes have been flying test runs over Hawai'i and Alaska since the Pentagon began rotating bombers through Guam in 2004. But they only started dropping inert bombs on the Big Island's Pohakuloa Training Area last month when the Air Force heightened the realism of the exercises.

In the past, the pilots only simulated dropping weapons over the Islands. Now, the pilots can see whether the bombs they release land where they are supposed to.

The planes are equipped to drop "smart" bombs, or weapons guided to their targets by global positioning system technology. But they don't use it in the Hawai'i drills.

Instead, the airmen rely on gravity — and extensive data on wind speeds and elevation — to deliver their unarmed bombs to the right spot.

Maj. Brian Bogue, deputy chief of strategy plans at the 613th Air and Space Operations Center, said such methods were extremely accurate and there was little chance any bombs would stray off the Pohakuloa range.

Planners intentionally pick targets that are in the center of the range, Bogue said, adding that two miles is the closest any of the bombs have come to the range boundary.

Further, as inert weapons, none of the bombs contains explosives, so there's no danger of one going off.

During their November training mission to Hawai'i, the bombers flew some 18 hours roundtrip. Ohio Air National Guard tankers refueled the planes in mid-air twice along the way.

During the last refueling session before the bombers headed back to Guam, a B-2 traveling some 400 mph gently eased up to a KC-135 tanker 26,500 feet above the Pacific Ocean.

When the bomber was just 20 feet away, the tanker attached its boom to the B-2 and started sending 35,000 gallons of fuel into the bomber's tank.

On the way back from Pohakuloa, the bombers launched a simulated attack on Pearl Harbor to practice targeting naval assets. Part of their mission was to use their stealth capabilities to sneak past their make-believe adversary's radar and take out its defenses.

"This particular mission covers the full spectrum of what we can do," said Maj. Tim Hale, one of the pilots in the exercise.

The B-2 bombers assigned to Guam also fly to Alaska for similar training exercises at the Yukon range. Their permanent home is Whiteman Air Force Base in Missouri, where all 21 of the Air Force's B-2 bombers are based.

The U.S. military started rotating bombers — including B-1 and B-52 planes as well as the stealth variety — to Guam in March 2004.

The move compensated for U.S. forces diverted to fight in the Middle East. And it came as North Korea increasingly upped the ante in the years-long standoff over its development of nuclear weapons.

In April 2003, North Korea told the U.S. it had nuclear weapons and might test them, export them or use them. Several months later, it declared it reprocessed 8,000 spent fuel rods from a nuclear reactor. Such a move, if true, would yield enough plutonium for a couple of nuclear bombs.

Bruce Bechtol, a professor at the Marine Corps Command and Staff College in Quantico, Va., said North Korea refers to the Guam bomber deployments in its propaganda, indicating it felt their presence.

Pyongyang realizes the United States would use the planes to respond if the North attacked South Korea, said Bechtol, who is an expert on air power on the Korean Peninsula. The isolated totalitarian state is also well aware of planes and forces the United States has amassed in Japan that could be used against it, he said.

"This all affects how North Korea looks at their foreign policy, how they look at the type of behavior they may engage in with their neighbor," Bechtol said.