The man beside me is screaming orgiastically. It is the poet and bon viveur David Gewanter. His eyes are wide open, his nostrils flare. This epitome of sober conventionality, a family man against whom no ill could fairly be uttered, is baying for blood, offal and giant ants. We are sitting among countless thousands of men, women and children caterwauling at the tops of their voices. The racket is deafening. Without warning, there sounds a terrifying roar from somewhere far beneath us. The crowd falls silent. My intestines clench, for I know I am about to bear witness to some vile, bestial obscenity. Smoke billows into our faces. This was what the gladiatorial combats and chariot races were like in ancient Rome, only what we are about to witness will damage our sensibilities for ever. Were a procession of wild animals and half-naked Spartan slaves to begin fighting before us, no one would bat an eyelid ... for this is the night of the Monster Trucks.

A colleague at the English department at Georgetown University, David is the eminence grise who has masterminded my introduction to American culture. We began with innocent visits to the Safeway supermarket on Wisconsin Avenue and graduated to local poetry readings. Exotic as these ventures were, nothing could have prepared me for the Monster Trucks.

Imagine the chassis of a pickup truck painted in Day-Glo orange and green, bearing the words "Grave Digger", mounted on 9ft-high wheels, each one of which has a retinue of custom-engineered shock absorbers. The 1,000-horsepower engine is mounted just behind the driver, runs on high-octane fuel and is capable of sounding an ear-splitting roar at the touch of a pedal. It doesn't matter who's at the wheel because the truck has a will of its own. When its name is called it shrieks back; when other vehicles trundle around the arena it quivers and pants, ready to eat them; and when summoned to battle it snarls bitterly, prepared to pounce. Each is unique, a car fetishist's dream. If you fantasise about fibreglass chassis, the chromium latches of quarter-lights, imitation wood laminates or the grotesque overhang of an instrument panel - Monster Trucks are for you.

But I have jumped to my conclusion; let me begin at the beginning. The Verizon Center is a huge arena in Washington DC. I've known of its existence since arriving in America a year ago and from time to time have strolled innocently past its walls. With no reason to enter, however, I never understood its vast size. This citadel can accommodate more than 20,000 people and is sufficiently large for a single circumambulation to consume nearly half an hour of your life.

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David and I arrive early for the Monster Jam, but it takes so long to reach our seats, which are on the opposite side of the building, that proceedings are under way by the time we occupy them. The Center is based, like all such venues, on the Colosseum in Rome. It heaves with a mob toked up on nutmeg, tape-head cleaner, PCP and ketamine. Blood-curdling, animalistic noises come from the upper reaches of the stadium, a sure sign that everyone is in a state of overstimulation.

And no wonder. This being America, the whole thing is run as a three-ring circus, a nightmare for anyone with a hangover but the ideal venue for those wishing to cultivate either deafness or attention deficit disorder. The overhead JumboTrons show magnified images of the violence below, repeated in slow motion whenever someone's life hangs in the balance; the deafening sound of heavy metal blares out of giant overhead speakers; wrap-around light systems flash words, images and other arcana across our line of vision.

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There can be no lulls; in those rare moments when no vehicle is airborne or taxi-ing to its doom, stagehands throw boomerangs at us. For some reason, those large red objects spinning perilously close to our heads get the adrenaline moving like a 1,000-volt blast in a copper bathtub and, within moments of arrival, both David and I are barking and braying with the rest of them. It is the revenge of the machines.

The overweight gentleman in the red polo shirt is the master of ceremonies. He is our Virgilian guide to the mysteries of the Monster Trucks and introduces each in turn, the names of which betoken panic, severe maiming and the Manichaean struggle between Good and Evil - The Broker, Superman, Viper, Thrasher, Screamin' Demon, Madusa.

The game is simple. Everything that happens in the arena is a variation on the conceit that the Monster Trucks, their supercharged engines spitting blood and sawdust, accelerate towards an uphill dirt ramp that propels the 20,000lb vehicles high above the crowd until they descend with a stomach-churning crash onto the cars, trailers and Winnebagos beneath their flight path. The trucks do this individually, compete to see who can jump the farthest, and then engage in "freestyle" riffs, ploughing everything in their path into shrapnel, coils of ruined scrap and dead meat.

As if that were not enough craziness for one evening, teams of quad bikers race round the stadium between heats - one representing New York, the other Washington. This agitates the crowd even more, with the opportunity to support the Washingtonians awakening all the tribal instincts built into the human psyche since prehistoric times. And then catastrophe: the New Yorkers win.

Mr Red Polo Shirt ill-advisedly hands the microphone to the victorious captain. "Hey, you assholes!" the New Yorkers' leader yells. "You're a bunch of wusses! Your team is shit, you're a lousy audience, and you're all gonna die!"

He is no Thucydides, yet his words speak to the crowd on a visceral level, many of whom are on their feet before he has finished his first insult, baying for his scalp on a pointed stick. "Rip his teeth out!" screams a voice behind me. The New Yorker is punched to the ground by the captain of the opposing team and they scrap in the dirt with the ferocity of sharks in a feeding frenzy.

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The stars of the show are the Monster Trucks. They have their own personalities, fan bases and capabilities. And the crowd knows that if they could scrap in the dirt, too, they would. The sight of each vehicle taking to the air, as if willing its own impossible transformation into a winged beast, has a delicacy about it similar to a prima ballerina in mid-flight.

Their ascent is remarkable enough - the sight of each of these ungainly, alien, clumsy objects as it climbs, swanlike, into the ether is breathtaking - but there is something stranger still. At the apex of its journey into space, Grave Digger hangs suspended for a moment, as if dangling from an invisible silver thread. "Thou wast not born for death, immortal bird!"

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The impossibility of it induces a giddy breathlessness among us. There's a moment's involuntary silence, and we realise collectively that we're witnessing something magical. This is the timeless moment of which poets write, the tracing of a vision conjured out of tinsel and sawdust that inscribes in air our deepest anxieties, aspirations and traumas. It is wish-fulfilment in its purest form.

No dream lasts for ever, and before we know it we are awakened. Poised over the carnage splayed across the arena, the Grave Digger - 15ft in the air - slowly begins to descend. The only sane response is to gasp in horror, for we know that what is to come will be dementedly nauseating. That's why you can hear so many screams. You can almost hear the sickening crunch before the truck makes contact with the twists and outlines of metal beneath it.

Of course, the landing could never be smooth - and it is always hard to watch. It is like a car crash, only ten times worse than any you could imagine. The Monster Trucks always come off best but, even so, their suspension is tested to its limit as gravity does its work, dragging them messily and frighteningly back to earth. From time to time the trucks come to real grief, and when that happens the effect can be similar to that of a medium-sized explosive.

The arena is soon devastated like the landscape of some forgotten planet. The Grave Digger is declared the winner - if, indeed, that is the word. For this has been a conflict out of which none could be said to emerge truly victorious. It has been a war of attrition. As David and I wander out of the Verizon Center, white-faced and exhausted, it is clear that other audience members are similarly affected. They stagger into the road like stroke victims; small children scream without comfort; on the pavements grown men throw up. Knowing what to expect, the city's police have shut down all surrounding access roads and summoned ambulances, lined up outside the main doors.

"This was not a psychotic episode," David assures me, gripping my arm a little too tightly. "We really did see that."

"Yes," I say. "I believe you."

As we walk out into the cold DC night, we know that the psychic scars will remain for ever. Those weird mechanical beings with a life of their own will run through our subconscious until the end of our days. And what can we do but embrace them? For, if nothing else, they are evidence of a reality higher than that in which we eke out our everyday existences, an exalted truth of which the sublunary world is the tackiest of imitations. And, curiously enough, I reflect, I'm in exactly the right state of mind in which to deliver tomorrow morning's class on Keats' Ode on a Grecian Urn.

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