The pool sat under a galvanised-steel pyramid. That was the detail that lodged in your head if you ever swam there. Faulkner Brown finished Bletchley Leisure Centre in 1974, a year when the British state still believed it could provide a tropical interior to a Buckinghamshire new town, and the answer it arrived at was a space-frame tetrahedron over a heated pool, glazed to the sightlines, surrounded by sub-tropical planting that the maintenance crew pruned during the night shift. The building came down in 2010, replaced by something with windows that did not need to keep the heat of the tropics inside an East Midlands winter.

There was a peculiar dread to those rooms even when they were new. The chlorine fog hit your sinuses in the entrance corridor and stayed with the rest of the day. The acoustics of tile and concrete turned a child's shout into a prolonged metallic ring that bounced off the roof and came back wrong, half a beat late, slightly higher in pitch. The fluorescent lighting was always too much for the eye and not quite enough for the camera, so every photograph from the period has the same washed-out yellow that makes the swimmers look as if they have been preserved rather than caught. You walked through a footbath of disinfectant before the water itself, and the smell of it was the smell of being small and slightly afraid.

Otto Saumarez Smith's Lost World of the British Leisure-centre Boom, published in 2019 in the Twentieth Century British History journal, makes the case that these buildings were the last serious municipal architecture this country produced. His phrase is "holiday atmosphere", lifted from the trade press of the period: pop imagery, fun, sun, the unashamed lowbrow populism of a council that thought a working-class family in Bletchley deserved to feel for an afternoon as if they were in Mallorca. Twenty-nine degrees centigrade, year-round, paid for out of the rates.

That model began to collapse the moment the funding mechanism behind it did. Coventry Sports Centre, the zinc-clad "Elephant" of 1976, closed in 2020. Swindon's Oasis, with its perspex dome, went the same year. Bradford's Richard Dunn Sports Centre, the Brutalist tent-roofed complex inspired by Kenzo Tange's Yoyogi gymnasium, shut in 2019 after a council decision driven by the cost of upkeep, and during the pandemic it served as a temporary morgue. The Twentieth Century Society's campaigns manager Oli Marshall told Dezeen in 2022 that every pool in the country was now at risk: the pandemic, the global chlorine shortage, the energy price shock, all of it falling on a building type designed in an era of cheap municipal gas.

What survives mostly survives by listing. The Oasis got Grade II status in 2021. Richard Dunn got it in 2022. The buildings themselves are mothballed, half-derelict, surrounded by hoardings, the tropical planting long dead. You can find drone footage on YouTube of the empty pools, the tile still gleaming, the ghost of a footbath at every threshold.

Five hundred leisure centres have shut since 2010, and the pyramid roof is a kind of period marker for the moment a place like Bletchley could still be promised the tropics by its own council. The dread of those rooms came from the gap between the promise and the building's own admission that the promise was provisional. The chlorine fog, the metallic echo, the yellow light, were the texture of an institution that knew, somewhere, it would not last.

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