One day, I met a young Russian lady who told me that one of her hobbies was swimming in an ice hole. My admiration was enormous.
How to come into this? In the past, she did whitewater kayaking, and all athletes had been used to ice water, because when a kayak flips over, it increases the chances of survival considerably if the athletes are used to the cold water. My admiration was boundless. I would never have done this in my life, and as a child, I would not have done it at all!
I could not stop thinking about the fact that this young lady could do something that was completely inconceivable to me. I started to be interested in this topic. On the Internet you will find a lot of material on this topic.

I found a video of a Russian diver named Natalia Avseenko, who (inter alia) had dived in the Arctic Ocean under the ice together with Beluga Whales and swam more than 10 minutes in the water - so twice the time that an untrained person can survive without taking serious damages.
Then I found out that in Eastern Europe many people dive into ice water, because this is a tradition of the Orthodox Church. On the day of Epiphany, they chopp (or saw nowadays) a hole into the ice, often in the form of a cross. The priests come out of the church in a procession and bless the water, and then many people enter the water, cross themselves and dive three times. This is to wash the sins of the previous year and bring health and blessings. Young and old are there, including grandmothers and children. You can find also many videos about this.
I thought, it can not be true that these children can do something I can not. But how can I achieve this?

Finally, I found a website where somebody described how he had managed this by taking showers and adjust the water a little bit colder every morning. This seemed to me a simple method to get used to the cold.
At the beginning of December, I started quite comfortably with 30°C. Every morning I set the shower half a degree colder. I always rinsed the legs twice, then the arms, then from the belly up to the throat and then the back. After that I rinsed the whole body and counted slowly to 60.
Shortly before Christmas I arrived at 20°C. It was already cold, but I had no problems, even if I had to breathe heavily in the first moment, when the cold water ran over the upper body. But that passed always quickly. However, it made me think that I had not yet been very far on the way from 30°C to 0°C. But I continued.
On the morning of 24.12. I showered at 19°C, and during this time I suddenly came to the insight: I did not get used to the cold at all! It was quite different: the cold worked like an anesthesia, so that the skin nerves were no longer able to pass the stimulus of the cold to the brain anymore. That's why my skin was always so insensitive when I was drying it with the towel. After I realized this, I turned the warm water completely off and showered at 14°C without any problems. I had no cooler water - so how could I continue?
On Christmas Eve, there was a relatively lukewarm breeze, about 14°C. I had to use this opportunity because cold air could demotivate me. I waited for the darkness because I did not want spectators with stupid questions, grabbed a thermos with warm tea and towels and cycled 5 km to a lake in the forest. The water was 4°C. I took off my shoes and trousers and went into the water. It was possible! Then I took off my shirt and crouched up to the neck in the water. That was also possible! I panted as always, but my breath calmed down quickly, I swam a very small round and dried myself off again. The skin tingled when it got warm again. I cycled home and drank the tea there.
Now I knew that I was able to do it because the few degrees would not make any difference . It even annoyed me that I had only stayed so briefly in the water. So the next evening I made a second attempt and swam back and forth until I had counted slowly to 60 as usual. No problems except clammy fingers.
On the next days I made two more attempts: I lay down in the bathtub at about 13°C. I had the feeling that this was harder for me than in the lake. Obviously temperatures between 10°C and 15°C are the most unpleasant. Below this it is getting easier because the skin becomes quicker numb, the colder the water is. That's why I was no longer afraid of the ice hole. I had the firm certainty that I can do it!

Even before Christmas I had found out where people can swim in ice water. In the area of Frankfurt there were only two options: the traditional New Year's Bathing Start in a swimming pool in Wehrheim (between Frankfurt and Gießen) and the more private New Year Bathing Start organized by by some nudists in the forest lake "Grube Prinz von Hessen" east of the Darmstadt.
I contacted these people because the forest lake is closer to Offenbach and because I imagined nicer pictures. The weather forecast promised slight minus temperatures and thus a thin ice layer, which would probably cover the lake, while in the pool the ice is always removed completely.
When I looked out of the window on the morning of January 1st 2017, I shivered, because everything was covered with hoarfrost. But that was exactly what made me hope for beautiful pictures. This morning I lft whe usual cold shower away. At noon I grabbed warm tea and towels, as well as my cameras with tripod. In the parking lot near the forest lake, a total of five men gathered, but one of them wanted only to watch.
The lake was completely frozen, which brought the organizer somehow out of the concept, because he did not consider it necessary to bring an ax. We went half way around the lake to the beach, and there was fortunately a small ice-free water surface.
I put my video camera on the tripod and pointed it to the edge of the ice, beautiful with ripe covered plants in the foreground. Then I gave my camera to the fifth man and took off my clothes. The three other men had already finished and dried off again. I stepped into the water and did just the same I had practiced: first the legs, then I dipped the arms, and then the rest. As expected, my breath calmed relatively quickly, and I swam to the place where I had installed the video camera. The ice was very thin and could easily be broken by hand. So I did not hesitate to make an idea come true that I had invented a few days ago: I took a deep breath, dived under the ice, and pushed my head up through the ice. That worked fine, even if the video recordings were not as spectacular as I had hoped. Then I swam back and dried myself off.

All in all, I had been swimming for one minute. I felt pretty good except that I had stiff fingers. The photographer had to help me to close the buttons of my shirt. Now the warm tea was really a relief!
Afterwards we went to a nearby restaurant, where had lunch with coffee and mulled wine.
The rest of the afternoon we spent in a sauna facility that drove the rest of the cold out of our bodies.

So the year 2017 began with the experience that one can still in the age of 63 years give up old habits and can overcome one's limits.
It is only a little disappointing that I am still a toe dipper and I am not inured at all. This was not a process of habituation, it was a learning process. But at least I don't have to see anymore that little children can do things which I can not do!