Don's diary: search for 'Palaeolithic Pompeii'

Tuesday
With my invitation and £120 in hand, I join the visa scrum and aim for Russia. I am off to examine an ancient volcanic deposit buried deep beneath the Steppe, in a layer close to where evidence of modern humans first appears. My preliminary work on this ash suggests that it was the most colossal eruption ever seen in Europe. Forget Pompeii; this was 50 times larger than Krakatoa. Its chemical fingerprint points to Naples'

Phlegrean Fields as the source. But to be sure, I need to see it in the field.

Friday
Evening finds us in the baroque splendour of St Petersburg: quite as glorious as the guidebooks promised. History assails us as we head along Moskovsky Prospekt: past Soviet-era iconography, stark memorials to the Leningrad siege, to the gaudy extravagance of the Tsars - freshly gilded for next year's civic tricentenary.

Russian hospitality is warm and welcoming, as Nikolai Praslov, our host and senior archaeologist, unveils his spectacular homemade vodka.

Saturday
We are headed for Kostenki, a rural settlement halfway between Moscow and the Black Sea. The village takes its name from the ice-age mammoth bones (kostya) found here, thought by Tsar Peter to be the remains of Alexander the Great's elephant train. Numerous Stone-Age sites have been excavated here since the 1870s, and it is now a key place for understanding human development. If our Neapolitan hunch is correct, the volcanic ash erupted about 40,000 years ago and marks a critical time-horizon in the sequence.

An ageing turbo-prop brings us to the city of Voronezh. Destroyed during the great patriotic war, it is now a sprawling industrial bustle, surrounded by black chernozem soils, the most fertile in Europe. The final stage of the journey brings us to a low, chalk downland. Home for the week is a rain-stained cabin; dusty works of Lenin and Marx line the rickety shelves.

Sunday
The Don slips quietly past below, as we stand on the first of two riverbank terraces. Evidence of ancient man is everywhere. Bones and worked flint fall out of the uppermost, ice age, "cultural layer". We need to go a little deeper to find the volcanic ash.

The site, Kostenki 14, lies inland on a promontory. Mounds of chalky soil flank the pit: a 20m square that plunges 7m down through ancient soils to the fifth - and deepest - cultural level.

Andrei Sinitsyn, the director, greets us. At his feet is one of the most extraordinary sites in Europe. I struggle to acquaint my eyes with the subtle colours of the sun-bleached section as my teenage summers of digging flood back.

Trowel in hand - I am not quite ready to wield the knives that my Russian colleagues deploy so deftly - the volcanic ash emerges 3m below the surface. This 5cm layer of soft, grey powder runs continuously round the site, clearly distinct from the chalky-clay soils above and below. The scale of this event is quite astounding: we are 2,000km from Naples, so this eruption would have choked 1 per cent of the earth's surface with ash.

Tuesday
We have now mapped and sampled the volcanic ash layer across site 14. In places, it is mixed in with wolf, rabbit and horse bones and crude flint scrapers. Cause and effect? We don't know yet, but it is natural to speculate. Andrei says: "This is a Palaeolithic Pompeii!" Below the ash, the fifth cultural layer is exposing astonishing quantities of horse bones, charcoal and flint tools. The first modern humans in Europe.

Thursday
The week closes with a double celebration: Andrei's birthday and the National Day for Archaeologists. Fortunately, an early flight allows us to escape the toasts with dignity intact. My heart stops as final boarding commences and the tannoy summons us to the desk. "You are upgraded to business class, gentlemen," we are told. Unwashed and unshaven: a Utopian end to an unforgettable trip.

David Pyle is senior lecturer in earth sciences, University of Cambridge.