Near the glass and the steel, a peacock glints blue and green

Most things happen by accident. Passengers go into Stansted airport either on the special train line which comes neatly to a halt in the concrete underbelly of Norman Foster's elegant terminal, or they drive along the pretty avenue of new trees, three miles long, from the M11 roundabout. The glass and steel of the terminal have the geometric assurance of a Euclidean proof.

You might think that everything at Stansted is the product of pure reason. Not so. That is Foster's conjuring trick. Take a left, instead of a right, as you drive towards his terminal, and you end up in the guts of the airport. The opposite end from the clear, rational face Stansted presents to the general public.

Pass the businessmen's Hilton (with its Living Well fitness club) and turn a wooded corner. You are now in a morass of sheds, of every age and size. Engineering sheds, customs sheds, sheds for strange airlines (for Air Harrods and for Laker, the battered forerunner of all cheap air travel). It was from here that an enemy of Idi Amin was being exported back to face the music in Entebbe, after he had been grabbed and drugged by Amin's agents. But suspicious customs officials opened the crate - supposedly full of Amin's favourite Scotch - he had been stuffed into.

Back here, the shabby little roads all have very grand names, from First Avenue up to Ninth Avenue (and maybe higher numbers, beyond where I can get to). The nomenclature explains some of the oldest, tackiest sheds - and also why London's third airport is at Stansted at all. This was one of the many American air force bases, laid out in the cornfields of East Anglia, ready for airmen to play their perhaps last-ever record of Artie Shaw before flying out to bomb Germany.

The abandoned (but very long) runways gradually picked up some cargo business. But for this, the third airport would have been somewhere else altogether. Ministry of Transport officials built a motorway to speed food lorries in from East Anglia. This made the choice even more certain.

Now, after only seven years, the Foster terminal is to be extended. Notices tell you Stansted is "Britain's fastest- growing airport" (up by 20 per cent a year). They also announce "the construction of new retail units". Foster gave the terminal its shape. But its contents are going the same way as any other airport: a mall offering "world-class shopping" to a captive audience.

If not world-class, Euro-class at least. Stansted is an EU dream, a gleaming advertisement for integration. It lives by short-haul. The next ten arrivals, flickering on the indicator, are from Dublin, Copenhagen, Bologna, Cork, Amsterdam, Dusseldorf, Carcassonne, Frankfurt, Kristianstad, and again Dublin.

As airports go, it is exceptionally discreet. Foster's terminal was designed to be hidden by trees. All I see, in the dusk, are a single plane's bright white landing lights. So far, Stansted is a dwarf by comparison with London's other airports: six and a half million passengers a year, against 29 million at Gatwick and 60 million at Heathrow. (But 15 million after the expansion.) Slowly it is changing the countryside around.

In the village of Stansted Mountfichet, little estates of new houses are being tucked in around the edges. In Bishop's Stortford, estate agents advertise a converted barn, handy for the airport and an "ideal company/corporate let": £3,000 a month, or nearest offer.

Bishop's Stortford is the kind of market town that, elsewhere, is dead on its feet. But there is no worry, around here, about the dying countryside. On a cold November Saturday, the streets are full of busy crowds. Couples are starting to make for the pubs. In the church on the hill, the East Herts Youth Orchestra rehearses its evening concert. The cheerful din of brass and woodwind bounces off the white walls.

An airport scatters noise around. With it, it scatters money.

I first came to the airport ten years ago, when Foster's terminal was beginning to spring up out of the Essex mud. Its marvellously engineered umbrellas of steel would be a return to the simplicity of the first days of air travel, when you just walked out of a shed to your waiting plane. The spirit, I suppose, of the original USAAF hangars. Behind all that high technology, Foster is a romantic: about air travel, at any rate. Speed plus light equals modernism.

But an airport is much more than a design image, even if the right image was probably the original intention behind the architectural commission. (As with Richard Rogers' design for the proposed Terminal Five at Heathrow, the thought was that it would help persuade the planners.) Business is business. Near Stansted, the landscape is almost all sky. As the sun goes down, the M11 slices through. A ribbon of electricity.

It is wrong, I decide, to be too romantic about this countryside. And yet . . . On my way out of Stansted Mountfichet, cars in both directions have to brake to a sudden halt. A peacock is walking slowly and beautifully across the road. It glints blue and green in the fading light.