Some are ancient mazes that run for miles, like giant anthills dug into limestone. Others are big, shallow, modern bunkers carved out of granite or other tough crystalline rock and reinforced with steel.
As the US military narrows its search for Taliban and Al Qaeda terrorist leaders including Osama bin Laden, one of its major challenges lies in outsmarting enemies it believes are hiding in Afghanistan's legendary labyrinth of caves and man-made tunnels.
If and when US forces pinpoint these leaders in their underground redoubts, the military has several options for "smoking them out" or eliminating them.
Perhaps the most likely lethal method, now in use in Afghanistan, is surprisingly simple: Seal off cave entrances using large bombs, or destroy the caves with giant 5,000-pound "bunker busters."
"Our specialized approach to caves and tunnels is to put 500-pound bombs in the entrance," says Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff.
US airstrikes have already closed up numerous caves, and more are regularly discovered.
Some military observers have suggested the use of other cave warfare tactics. For example, one deadly approach would be to vacuum air from the caves.
First, a bomber would drop a bunker buster on the cave to fracture the rock. Next, a "daisy cutter" could be exploded above the surface to extract the air, depriving anyone inside of oxygen, says J. Kelly McCann, a former US special operations officer who is now president of Crucible Security in Fredericksburg, Va.
If the military seeks to capture the enemy alive, it might try to drill airshafts into the cave and release chemical gases to immobilize those inside. "You deliver immobilizing gas, then walk inside," says Jack Shroder, a University of Nebraska geologist with decades of experience in mapping Afghanistan and its caves.
Other experts, however, suggest that these methods are unrealistic, and that sealing off the caves is the most likely technique. Then troops would encircle the entrance and wait for any of the enemy to emerge.
One point of general agreement is that the Pentagon, at least for now, probably won't send US troops into the tunnel complexes on a risky, cave-by-cave hunt. "We don't have plans to go into caves from the ground," says Defense Department spokesman Lt. Col. Dave Lapan.
Instead, Defense Secretary Donald Rumsfeld has said he hopes that the up to $25 million rewards offered for Mr. bin Laden and his lieutenants will inspire Afghans to "begin crawling through those tunnels and caves, looking for the bad folks."
An even greater challenge than attacking the tunnels, experts say, is pinpointing the Al Qaeda leaders' location.
Since mid-November, the routing of Taliban forces from much of Afghanistan has significantly boosted US commanders' hopes of both tracking and, if found, striking bin Laden, Taliban chief Mullah Mohammed Omar, and other leaders in their hiding places. Loitering US warplanes on Tuesday bombed a leadership compound southeast of Kandahar, where intelligence indicated Mr. Omar might be, although Taliban officials said later that Omar was unharmed.
Gen. Tommy Franks, the US commander of operations in Afghanistan, said Tuesday that both human intelligence and imagery was leading the military to hone in on two regions: Kandahar in the south, and the mountainous corridor from Kabul and Jalalabad to the Khyber Pass in the east. A large portion of US airstrikes are now hitting at cave and tunnel complexes discovered in those areas.
Any hunt should be aided by the "forward operating base" established by the 800 to 1,100 marines who landed this week in Afghanistan. Their control of an airfield 70 miles outside Kandahar should dramatically shortened - from several hours down, presumably, to minutes - the time it needs to launch airstrikes at targets pinpointed by intelligence.
"We're interested in the destruction of the al Qaeda network, and ... the leadership of the Taliban. Now we can either do that ... by making seven-, eight-, and nine-hour trips, or we can provide ourselves a forward operating base," Franks said.
Earlier this month, Secretary Rumsfeld expressed exasperation that reports locating bin Laden were "always late."
As the Pentagon continues "tightening the noose" on bin Laden and other Al Qaeda leaders, it is using a range of tools to narrow down which of the hundreds of cave and tunnel complexes are likely hiding places.
These include satellite imagery, unmanned aerial drones, and the military's latest high-tech sensors that can detect heat and possibly vibrations of machinery. Also being used are US reconnaissance troops on the ground, intelligence and maps from Russia and Pakistan, as well as tips from local Afghans.
While there is no guarantee that bin Laden is holed up in a cave, US officials have repeatedly stated in recent weeks that senior Al Qaeda leaders have been using the cavernous dwellings, although they frequently move from one to another.
"Bin Laden sends out decoy truck convoys, day and night - it's the old shell game," says Mr. Shroder, who has advised the US military. His best guess is that bin Laden is in one of the large complexes of reinforced, man-made bunkers blasted or chiseled out of hard rock near Jalalabad.
In the 1980s, bin Laden was active in acquiring heavy machinery to help make such tunnels, according to John Pike, a military analyst with the firm Global Security in Alexandria, Va.
"These are steel-lined, hardened bunkers," Shroder says. "They contain food, places to sleep, weapons." Tanks, armored personnel carriers, and Toyota trucks can drive into the complexes, which sometimes contain internal sources of seeping water as well as places to dispose of human waste.
All this allows troops to stay underground for extended periods. "People can hide in caves for long periods, and this will take time," Mr. Rumsfeld says.
US commanders admit the danger exists that bin Laden could flee, perhaps over one of the more than 150 small, often rugged, paths leading from Afghanistan to Pakistan.
But Gen. Franks vows to pursue the leaders into other nations. "If this leadership does come from Afghanistan, it's simply a matter of continuing wherever they go until we find them," he said. "We surely will."