The cave complex, overlooking the village about 35 miles [56 kilometres] south west of Jalalabad, was used by mujahideen forces fighting Soviet troops during the 1979-89 war.
Heavily fortified, the base was built in existing caves with covert assistance from the US. Russian troops failed to destroy the complex, which has been described as "the last word in Nato engineering".
The village of Tora Bora itself, which lies in the meeting place of two valleys in the deep mountains of Nangarhar province, has been described as "virtually impregnable".
Reports, which could not be independently verified, said Bin Laden had been sighted in the area travelling at night by horseback and protected by 2,000 heavily-armed al-Qaeda fighters.
The fortified caves are thought to contain ammunition and storage depots for sheltering, supplying and replenishing a guerrilla army. The caves are said to be heated against the harsh Afghan winter, and have their own ventilation systems.
An arsenal of of US surface-to-surface 'stinger' missiles, which were extremely effective against Russian armour during the 1980s, is also believed to be stored there.
Remote location
The base, which is surrounded by deep forests, can only be reached on horseback. The main access to the complex is via a series of mule trails, which are primarily used as supply lines from remote northern Pakistan.
The US has been targeting the area with GBU-28 bunker busting bombs, which can 'burrow' up to 100ft underground before detonating.
However, it is thought that the construction of the caves, with rooms located at right angles from a main access tunnel, protects the main areas from missiles.
Unmanned 'Predator' spy planes equipped with heat-detecting sensors have been scanning the area searching for tell-tale heat emissions from the caves, which remain warm as surface temperatures fall.