I knew the street of old. I had passed along it a hundred times but never noticed the entrance. The gate was easy to overlook: high, wrought iron and set back from the pavement between two lofty Victorian townhouses. Nothing drew the eye to it or the narrow brick passageway that yawned away behind it into the shadows. That first Saturday morning I lingered there at the gate for a moment, feeling its metal pickets warm in my hands, gazing towards the end. On an adjacent wall hung a rusted, century-old Hungarian drinking fountain. Above the basin were the words Ülj le velem, énekelj nekem, inni tőlem. Sit with me, sing to me, drink from me. Strange, I thought, in west London.

I knew the code and typed it into the keypad. The lock clicked open. I stepped through the portal and the sounds of the city – a gear-grinding number 9 bus, tennis matches in Holland Park, a tantrum of wailing toddlers at a nearby playground – fell away with the light. I heard my footsteps echo off the walls. The passageway seemed never to have seen the sun. Neither a blade of grass nor gathering moss grew on its scrubbed flagstones. At its end my destination was hidden from the street. The squat and plain building had no street number, no other entrance. I held the key in my hand. At a large white door, heavy with age and humidity, I turned it in the lock and felt the tumblers drop into place.

I stepped out of the dark and into a high, bright sanctuary. Sunlight fell in slanting streaks. On dozens of shelves and desktops rose teetering piles of old photographs and dusty metal tins of transparencies and film. Tall wooden racks were crammed with thousands of numbered manila folders, stacked at wild angles like something the earth had thrust up, a geological slice of civilisation. I moved between them as if through tunnels, into history, within a labyrinth of corridors and pathways shaped from floor to ceiling by more columns of photographs. I ventured downstairs and stared in wonder at a basement that was crowded with pictures. I wandered through the image galleries. I brushed my hand across the bulging albums. I paused and picked up a print at random, turned it in my fingers, felt the paper’s weight, smelt the distant hint of chemicals, ink and transience. Once the building had been a London family home. Now it was a secret place unhitched from the stream of time: silent, poignant and deserted, yet peopled by the past.

My agent had been sent the key along with an invitation that I could not refuse. I was to have the run of the archive, to do there what I wanted for as many weekends as I cared to do it. The offer spelt out no specific commission or conditions, apart from the insistence that the archive’s curator not be named. Online all I’d been able to discover about him was that he was a very private person, not short of a few bob, who over the last 25 years had collected photographs. Four million photographs. I’d learned that the amateur snapshots, press prints, throwaways and daguerreotypes had been plucked from all over, from Berlin flea markets and Beijing recycling plants, snatched from defunct Soviet institutions and bankrupt British publications, bought on eBay and at small town auction houses. My mysterious patron had created one of the world’s most moving image treasures, and told virtually no one about it.

I hung my jacket on the back of a chair and began to open files, to peer into albums and shuffle through boxes of loose negatives. Serendipity guided me at first, letting me travel across the eras and ages. I picked up a photograph of a Toronto birthday party at the start of the Great War and saw uniformed men in bushy moustaches raise their glasses to swift victory. Their faces radiated confidence and hope. God Save the King echoed off the walls of the old University Avenue Armouries. I ventured into the next room and found myself kettled in a shattered Stalingrad cellar with a dozen broken Wehrmacht soldiers. In faded Agfacolor prints the men huddled in the bitter cold, picking at their lice, crushing them between fingernails as Russian guns closed in for the kill. Next in black-and-white I rode alongside an unknown Vietcong tank driver at the fall of Saigon and then lingered over the boorish scrapbook of an anonymous KGB operative-come-pornographer in West Berlin.

Time and again, while holding a child’s giddy snapshot or intimate lover’s portrait, I was moved beyond words – and then to words – by these glimpses of past lives. I understood quickly that the archive was no unknown thing, no terra incognita. In fact it was quite the opposite. Each album, every snapshot, every precious file was the record of a life, perhaps the only surviving evidence that the individual had ever existed. I realised that the vast collection was a kind of lexicon of lives – a compendium of countless actions, thoughts and emotions that I had been given the chance to discover…