It started with a dare. Three of us, close friends, were sitting around a table on Christmas Eve three years ago, and the subject of the Highgate Women’s Pond came up. I said I’d never been in it. One friend’s husband said to me, ‘Well you’d never manage to get in during the winter’.

That of course, was that: a red rag – come Boxing Day morning, the three of us were poised anxiously on the edge of the pond, egging each other on. It was a mild winter, thank God, and the water temperature a relatively balmy 9 degrees. Somehow we got up the nerve to lower ourselves into the icy water, as the wonderfully kind and patient lifeguards watched on.

The shock was intense, but then so was the excitement and the extraordinary endorphin rush that followed. We were high on the joy of it and the cold was like a tonic. The pond was beautiful in the pale dawn. Other women were there, calmly swimming up and down as though it were a balmy afternoon. The next week we were back, as a foursome. The week after that, five – word had travelled around our north London neighbourhood.

We have got to know each other through the highs and lows of life, and with the shared intimacy of the swim comes a rare camaraderie.

Later that first winter the pond froze and the aerators failed, so we had to lower ourselves into the water and swim around the ice, which tinkled as we drifted past. The highs grew higher; the group grew larger. The seasons changed, spring arrived, and with it goslings, moorhens nesting, the odd cormorant watching on regally from the buoys dotted around the pond. With the warmer weather came the removal of the rope which halves the water in the winter and the endorphin highs were achieved now by staying in longer. And then the summer mellowed into autumn, we swam through leaves that lay on the glassy surface of the water, the temperature dropped, the rope went back across, and soon there was the first frost, and we had swum full circle; our first year at the pond had elapsed.

The group has continued to grow – now, three years later, there are around twenty of us. We are loosely connected as women of a certain age with hectic careers and children, who live within in a mile or two of each other. We have got to know each other through bereavement, divorce, illness, the highs and lows of life, and with the shared intimacy of the swim comes a rare camaraderie.

The weekly pond mission has developed into bigger adventures – an annual swim, down the Thames one year, in the Lake District another. There is a plan to swim 10 km down the Dart River this September. But the weekend visit to the pond remains at the heart.

Above all, there is the peculiar beauty of the place we visit, an oasis in one of the biggest cities in the world.

One of the three musketeers from that first Boxing Day swim organises us by sending round an email each week. Women dip in and out depending upon their busy lives. It is for all of us something akin to church – the regularity of shared ritual in the early morning light, the lifeguards who officiate, keep us safe, administer cups of tea and blankets when one of us stays in too long.

Above all, there is the peculiar, magical beauty of the place we visit, an oasis at the top of the heath in one of the biggest cities in the world, delight in seeing the seasons change, week by week, and the extraordinary sense of wellbeing that goes with each visit – pure sensual pleasure and vitality.

I think I speak for all of the wonderful women with whom I take the plunge each week when I say that hidden world of the Highgate Women’s Pond has changed my life utterly, and for the better.