Late last year, after a decade of failed attempts to make the journey, I drove 14 hours across Afghanistan to look upon one of the architectural wonders of the world. The road east from Herat had long given way to dumpy tracks and rocky riverbeds when suddenly - through a cleft in the looming cliffs - I caught my first glimpse of the minaret of Jam, a fragile sliver of tapering, man-made beauty dating from the 12th century and set among momentous mountains.
Rising 65 metres, the minaret is far from inconspicuous, yet such is its remoteness that rumours of its existence only reached the west in 1944. The tallest complete and authentic ancient minaret in the world, it is believed to have been built by the once great Ghorid empire, who in the late 12th century ruled over what are now Afghanistan, Pakistan and India, as far south as Delhi. Before a French archaeologist finally located it in 1957, the minaret had endured more than 700 years of obscurity and neglect after the Ghorids were defeated by Genghis Khan.
Now this moving, beautiful building is once again under threat. In 2002, Unesco declared Jam a World Heritage site, the first in Afghanistan, but soon after also listed it as a site in danger. Rich in archaeological remains, the spot has been ruthlessly looted for the past 20 years, while the minaret itself is slowly crumbling and tilting, its foundations undermined by the waters of a nearby river. Repairs started in the 1970s, but were effectively halted by the conflict that subsequently engulfed the country. It was only in 2001 that work to stabilise the minaret and shore up its foundations finally resumed.
But these repairs have now halted again, and visits from the outside world have all but stopped. The journey to Jam is deemed too dangerous for most, with threats ranging from local banditry to abduction or execution by insurgents who range through Ghor province, in which the minaret stands, on their way south to do battle with the International Security Assistance Force (Isaf) in Helmand.
Yet, with caution and careful preparation, a trip to Jam is possible. My own journey was with a team from the BBC; we hoped to be the first crew ever to make a TV programme about the minaret, and in so doing remind the world of its wonder and vulnerability. Our journey was complex, arduous and at times baffling. When we finally arrived in Herat from Kabul, we had to gain permission to travel from various Afghan authorities, as well as clear our plans with the Isaf.
Meetings with the deputy governor of Herat and his chief of police went well, but we then needed permission from the chief of police for the western provinces. This was crucial. After questioning, this powerful policeman decided we were a good thing, and took us under his personal patronage, giving us an escort of 60 police for part of the journey. Flattering, perhaps, but our convoy was clearly going to be more high-profile than we had wanted - and in Afghanistan, the police are targets for insurgents.
Then we met the Italian army, the Isaf body responsible for security in Herat and Ghor. They were impressed by our plan and, perhaps, our audacity - they had never sent a patrol to Jam. In fact, none of the Afghans we met in Herat had made the journey, either.
But we managed it - and it was worth every difficulty. The minaret is made of hard yellow brick. As I stood before it, I saw that it is in this material that the building's most striking message is written. Its shaft is a dazzling display of virtuoso brickwork, with geometric forms incorporating Islamic eight-pointed stars and Kufic lettering. There is a panel bearing the date of construction: 1193/4. But, more importantly, the lower portion bears the entire 19th sura of the Koran. This chapter, called Maryam, tells of the Virgin Mary and Jesus, both venerated in Islam, and of prophets such as Abraham and Isaac. It's a text that emphasises what Judaism, Christianity and Islam have in common, rather than their differences. It seems the Ghorids placed the text here to appeal for harmony and tolerance in the land, a message that is more relevant now than ever.
Inside, I found a stupendous, engineered construction, with two spiral staircases winding around each other to form a double helix. This strong construction, combined with the fine brickwork, has preserved the structure from earthquakes and neglect. But for how long? As we filmed, the police became increasingly unsettled, fearing "enemies" might be gathering against us. So, sooner than I would have wished, we left the minaret, solitary and exposed in its remote valley. For more than 800 years it has survived against the odds - but all who care about it must now tremble for its future.
· The minaret of Jam features on Dan Cruickshank's Adventures in Architecture, tonight on BBC2.