On a foggy morning last spring, I took my brother and 11-year-old niece for a long walk in one of Britain’s oldest nature reserves, Wicken Fen, a tiny fragment of a lost marshland ecosystem that once covered around 2,500 square miles of eastern England. We spent hours walking through a mosaic of grassland and sedge, past wet fields cut with scrub-shadow and flashes of open water. Everywhere was bursting with life: singing nightingales, snipe winnowing through the upper air, cuckoos tilting from the tips of willows and rails squealing and grunting in reeds. As we walked on wet grass by one of the fen’s ancient waterways, a barn owl floated past us, its mothy wings shining through particulate mist; at our feet a drinker moth caterpillar inched furrily across the path like a cautiously mobile mustache. We knelt to watch its progress. Then my niece turned to me and asked: “When they made this place, where did they bring the animals from? Did they come from a zoo?”

I have been coming to Wicken for many years, bewitched by its strangeness and beauty. And today, as I walk its muddy paths under drifts of pallid cloud, my niece’s inability to understand that the life that is here was once everywhere still disturbs me. But that is, after all, why we’d come: Nature reserves are places to experience the past — the British environmentalist Max Nicholson once described them as outdoor living museums. Fen landscapes are unstable places where familiar categories of water and land are disconcertingly confused, and they feel temporally unstable, too, rich with a sense of their layered ages. Walking in them is an act of virtual time travel.

As I walk, I think of the natural wealth of the 11th-century fens, where fish and wildfowl existed in such astonishing plenitude that local debts were settled with payments of eels — fish-silver — and Saxon warlords hid in the swamps from Norman invaders. And I think of 17th-century villagers who were at home here, who cut sedge and reed for thatching and dug peat for domestic fuel. In the 19th century, naturalists flocked here in search of insects. So many people brought lamps to attract moths at night that there were complaints the fen looked lit by streetlights. Darwin collected rare beetles from Wicken reeds that were sent to Cambridge in boats to light university fires, and sugar-smeared poles stuck in the ground to draw moths to their sweetness took root and grew into today’s vast trees.