It is dark and the water glows aqua under the spotlights. The red, white and blue flags are motionless and the lanes are busy. This is a pool for devoted regulars, for families, for couples doing handstands and determined lane swimmers, tired bodies reclaiming a piece of themselves after the mental exertions of the day. I join the natural flow of bodies moving in a circuit in the water. The pool is heated but the air is not cold enough to produce steam. The lights in the sides of the pool create beams of white that dance and ripple as you swim through them.

On the side, there’s a children’s birthday party, gathered parents singing. The water glitters in the evening, and there’s lots of chatting and laughing. Overhead, the planes rumble close and low. Swimming, I turn and look up every time one passes, lying on my back to watch the night sky. Then I find my way out of the lanes and into the free swimming section. I sink down and sit on the bottom and watch the underwater world.

When I come up, it is as if the water swallowed the light. From the purple, unsteady light of early dusk, the sky has deepened into midnight. Under the water, a hundred shifting, dancing reflection lines are creating impossible, never steady shapes. I stretch and let the air nip at my wet skin. I get out and wrap myself in my towel and stand at the edge, watching the swimmers circling in the under-lit water.

I’m raising money for the Alzheimer’s Society. Please sponsor me at http://www.justgiving.com/swimbonnieswim