The border mountains provide many escape routes
US Secretary of State Colin Powell has admitted there is no fresh information about the whereabouts of Osama Bin Laden.
Afghan commanders leading the operations against the Tora Bora cave complex have said they, too, have no idea where the Saudi dissident and any al-Qaeda fighters who have fled might be.
US President George W Bush has once again promised that he will be found.
But the geography and ethnic make-up of the areas which form the border between southern Afghanistan and Pakistan are real obstacles.
The trail of Osama Bin Laden has gone cold, and there is a real possibility that he has fled eastwards into the border areas with Pakistan.
The sheer scale of the border region and the harsh terrain have led one US military commander to compare the hunt for Bin Laden to "searching for fleas on a dog".
These mountainous areas are lacking any real roads although their elaborate networks of high, winding paths have been used for decades for the smuggling of arms, drugs and people.
The border is largely unguarded, and passing from Afghanistan into Pakistan presents no real difficulties.
Pakistan has stepped up border patrols and is co-operating with US special forces in locating al-Qaeda fighters on the move.
Local autonomy
But it is not just the physical relief of the border areas that means locating small groups or individuals is very difficult.
The Pakistani side of the border is home to largely autonomous areas where the Pakistani state is reluctant to assert control by force.
Local traditions are strong, as are the over-riding religious loyalties among the Pashtun tribes, some of which straddle the border with Afghanistan.
These loyalties mean that village elders would probably provide refuge.
But the arrival of strangers in such a village could not pass unnoticed.
And with the US maintaining its multi-million dollar reward for information on Bin Laden's whereabouts, Washington will be hoping that prize will outweigh traditional religious conservatism.