As the US peels back layers of Saddam Hussein's brutal regime in Baghdad, one dangerous level may remain: subterranean tunnels and bunkers, or "terra incognita," as one former CIA operative calls them.
The US has been targeting these labyrinthine hiding places with airpower since the first day of the war. In what may be the war's last frontier, CIA paramilitaries and Special Forces units will engage in the combat equivalent of an archaeological dig, searching underground for any remaining leaders of the regime, plus documents, WMD, and military materiel - including combat jets and missiles - he may have squirreled away.
They'll be helped by a rough map of underground Baghdad developed over the years by US intelligence. In part, this information comes from blueprints provided by the foreign construction firms that built many of Saddam's government buildings.
One remaining question: how extensive are the catacombs? For years, rumors have swirled in intelligence circles about subways or extensive tunnel systems under Baghdad's streets. So far, the US has no confirmation of these rumors.
"Clearly, there are bunkers, and some of these bunkers are linked to other buildings, but there is no evidence to substantiate that there's an elaborate system beneath the city," says a US official with access to intelligence on the subject.
Bunker war is one of the most dangerous and unpredictable forms of close combat. In Vietnam, the North Vietnamese and Viet Cong created elaborate networks, virtual underground cities, and took a heavy toll on US forces that dared attack them.
In Afghanistan, Taliban and Al Qaeda bunkers protected forces from most US bombing raids, and prolonged the ground fight. But in that country's relatively open terrain, US forces could use "bunker buster" bombs and other explosives with impunity, quickly reducing many of the bunkers.
In Iraq an attack on a suspected leadership bunker was the first act of the current war. To this point, the US has continued to target Baghdad bunkers with 2,000-pound bombs that explode deep underground, reducing vast areas of concrete to rubble.
Whether this effort has killed Saddam or any other member of Iraq's senior leadership remains unknown. And now, with much of Baghdad in its hands, the US is likely starting the second stage of attack on these redoubts.
This doesn't necessarily mean sending in Special Forces teams to explore - at least, not at first. It could involve staking out known exits and entrances, for instance. It might involve sending in remote-controlled robots.
"You attach a camera and a light [to the robot] and it records whatever is in there, so you can know before you wander in what you're likely to see," says retired Brig. Gen. John Reppert, a military analyst at Harvard's John F. Kennedy School of Government.
Bunkers are the more searchable underground target. Often built with large blast-proof doors, they offer easy access. And they're not extensive: Searching one is like searching a deep basement.
Tunnels are another story. There the defender can remain quiet, while the attacker must make noise during advance. Nor would US night-vision equipment provide an advantage. Underground, there is so little light that such technology can't be used.
Yet the US will have to sweep these areas fairly quickly, as they are thought to be a primary hiding place for top Hussein officials. US officials have said they cannot claim outright victory until these people are captured or killed.
And such a claim "would be later rather than sooner," Defense Secretary Donald Rumsfeld said at a briefing this week. "I don't think it would necessarily hinge on Saddam Hussein." He went on to say that victory would hinge on utter US control of the country, when freedom of movement is guaranteed, humanitarian aid can be delivered, and displaced Iraqis can return home. He also said it would involve finding any caches of weapons of mass destruction and the documentation of Iraq's WMD programs.
That documentation and other secrets of this regime may well be hidden in fortified bunkers beneath the lavish presidential palaces and other buildings in the capital. What's more important: Hussein and his sons - if they miraculously survived the latest bombing aimed at ending them - may have escaped, as they did on March 19, through the tunnels that link these bunkers. Other members of the regime on the US's "dirty dozen" list, as well as top Baath Party officials, may be squirreled away as well.
There's no way that the US wants any of these figures to emerge, or even feign control over the Iraqi population - moves that could prevent Iraqis from welcoming a new regime. "Part of the whole campaign is to remove the Iraqi leadership," says a senior US official. "There are segments of the population who, as long as they believe Hussein is alive, will be in no hurry to rise up."