Three thousand years before Tolkien was born, people came to the banks of Coldbath Brook to sweat. Archaeologists have found burnt mounds here, piles of cracked stones and charred wood that mark the remains of prehistoric sweat lodges. The pattern is familiar from Bronze Age sites across the British Isles. Water was poured over heated stones inside a small hut, the air filled with steam, and the bathers emerged to plunge into the cold brook. Whether the purpose was spiritual, medicinal, or simply social, no one can say with certainty. What survives is the residue of fire and water, now protected as a Scheduled Ancient Monument. People have been using this hollow for ritual or relief for longer than England has been a country.

Most of England's primeval wildwood was cleared before the Domesday Book was compiled in 1086, the trees felled for meadow and common land where livestock grazed and farmers worked strip fields. Moseley Bog escaped. The ground was too wet, the slope too awkward, and somewhere along the line someone decided the trees were more useful as a reservoir to feed the millpond at Sarehole Mill, a short walk downstream. The embankment built to hold the water still runs along the eastern edge of the reserve, though the bog was drained long ago. Coldbath Brook flows on, through a culvert from Moseley Golf Course, through the wood as an open stream, then underground again to the mill. The oaks above it are the descendants of a forest older than written English.

In 1980, Birmingham City Council announced plans to build a housing estate across the site. A local activist named Joy Fifer organised the resistance. She gave the place its modern name, Moseley Bog, in place of The Dell, and her Save Our Bog campaign drew on Tolkien's growing posthumous fame to argue that this scrap of suburban woodland was a national treasure. The campaign succeeded. The bog was saved, and Fifer's effort became one of the founding moments of the Urban Wildlife movement, which insisted that nature in cities mattered as much as nature in national parks. In 1987, the adjoining school playing field was planted with trees and allowed to revert to woodland. It is now called Joy's Wood, after the woman who refused to let the bulldozers in.

Tolkien lived at 5 Gracewell Cottages, on what is now Wake Green Road, from about 1896 to 1900. He was a small, bookish child with a sharp eye, and the rural Worcestershire landscape made an impression that never faded. In a 1966 Guardian interview, he named Sarehole Mill and its surrounding countryside as the inspiration for the Shire. The Mill at Hobbiton, where the Sackville-Bagginses came snooping, is Sarehole Mill, rebuilt in prose. The dark wet wood Frodo loses himself in is the bog. There is a literalism to Tolkien's imagination that scholars sometimes underestimate. He did not invent Middle-earth from nothing. He remembered a place and translated it. The trees you walk under here are the same species, in some cases the same specimens, that a four-year-old peered up at.

A house adjoining the reserve, since demolished, served as the rehearsal studio for the earliest recordings of UB40, the reggae band that took its name from the British unemployment benefit form. Their first sessions happened here, in a Birmingham backwater inspired by an Oxford don's prose. Since 2011, the bog has hosted the In Memory of a Free Festival each summer solstice, the title borrowed from a David Bowie song. Bronze Age sauna, Saxon clearing, Victorian reservoir, Tolkien's childhood, reggae studio, solstice festival. Twelve hectares of mud and oak in a Birmingham suburb, layered like geology, and still functioning, still loved, still wet.