The effort to catch Osama bin Laden, the suspected mastermind of last week's attacks on New York and Washington, is not only expected to take his pursuers to the other side of the world, but into the bowels of the earth as well.
Much has been made of the rugged, hilly terrain of Afghanistan, but bin Laden is also expected to take advantage of the country's extensive network of tunnels, an aspect of the terrain that can further tip the scales in favor of those who know the land.
The karez, as they are called in the local language of Pashto, were originally built to give Afghans a way of preserving water during the dry seasons. They have also become a defensive asset and rampart, helping repulse invaders for hundreds of years during war times.
According to some sources, the tunnel system is larger and far more extensive than the underground networks used by the Viet Cong in South Vietnam.
"They're pretty important," said Luke Powell, a nature photographer who has worked extensively in Afghanistan. "They're a major part [of civilian and farming life] in the south."
The Taliban is also strongest in the south.
Since they are used to transport water, the karez are usually close to occupied areas, and often serve as a ready-to-use shelter for both villagers and soldiers.
"When they go underground, they're a dandy place to hide," Powell said.
An Ancient Defense System
Some historians say the karez have been in use as early as 300 B.C., when Alexander the Great went through Afghanistan.
They've been a strategic advantage for the Afghans ever since: from 1224, when Genghis Khan's Mongols invaded the country, to 20 years ago, when the Soviets did the same.
During the war with the Soviets, the underground passages allowed Afghan rebels to besiege the town of Khost for almost a decade.
Khost's defenders tried Scud missiles, carpet bombings, artillery fire, helicopter gunships, and commandos, but nothing short of nuclear weapons could drive the rebels from their positions in the surrounding mountains.
The town survived the attacks until 1991, when the rebels poured from their encampments and overran the city.
Seven years later, the tunnels continued to prove their worth against an attack from the United States.
In August 1998, President Clinton launched 70 cruise missiles at suspected terrorist camps near Khost and another town, Jalalabad, in retaliation for the attacks on U.S. embassies in Nairobi, Kenya, and Dar es Salaam, Tanzania.
But the missiles' main target, bin Laden, survived. Some reports said he wasn't even in the area. Others said he escaped with the help of the tunnels near Khost.
Bullets and Bulldozers
That a structure used to defeat the Soviets should also stymie the United States is not the only irony to be found in the tunnels of Afghanistan.
Many of the tunnels are also believed to have been built or fortified with the help of Western money.
The exact source of the money is vague. Some reports have said the CIA. Another account, which appeared in The New York Times shortly after the U.S. rocket attack on Khost, described the rebel camps there as "the last word in NATO engineering techniques."
There are suspicions that bin Laden didn't even know where the money came from, because it was funneled through Pakistan's intelligence agency, the ISI.
But what is known is that bin Laden, son of a Saudi construction tycoon, and himself a civil engineer, would have known how best to build and take advantage of such a system.
Landscape and Character
Whether or not the karez systems have been fortified and militarized in the course of Afghanistan's many conflicts, visitors to the country say they are an unmistakable and characteristic part of the landscape.
People who have flown over the country say they can see them, neat little mounds leading across the desert to green zones of vegetation surrounding towns and villages.
Powell, describing the wells that begin most of these karez, added the tunnels were noticeable from the air in another way.
The wells dot the landscape, he said. "It looks like machine-gun fire."