It doesn’t look like the grey blocky exterior of park road leisure centre will hide a leafy oasis, but when I get through the warren of perfectly average changing rooms, I find a long blue pool edged with grass and screened by tall fir trees. You would never know the busy roads of North London are just outside.

The 50 metre pool is a deep, summer-holiday blue, not the aqua I’ve come to associate with pool liners and with lidos. Paired with the green of the grass, the deeper shade of the trees, it feels subdued, but concentrated. The day is grey, cloudy, and I have no doubt that in sunshine it would be heightened. The weather has its benefits – despite this time slot being advertised as a family play session, there are only three other people in the water.

I swim up and down, delighting in the long stretch, the flat placidity of the water, the steady monotone of the sky. The water’s surface is cut into triangles that shift and undulate, flowing into each other with the movements my hands make. All the shades of blue ripple and rise together. The pool is shallow at either end, with a dip in the middle and solid black lines marked on the bottom. It is a definitively controlled space, not a natural one, but being beneath the sky opens it up, barriers to each side and below but nothing up above. On my last length the sun comes out and paints shifting rainbow-yellow shapes underneath the water.