Prof. John is an athletic man. He said that he used to ski a lot in the winter. Not just normal skiing. He did Cross Country Ski. That means he opens up new paths in the snow, up & down the hills, make sure the snow are compact enough to for support.
Try opening ski path in places like this.
Those who have ski would know how hard it is to open a new path or going uphill. And John would do this everyday in the winter, same path, during the lunch time, alone. It is very dangerous to divert away from the path you created because you might fall into deep places which couldn't be seen in the snow.
One day few years ago when it was almost spring, he said he has diverted away from his original path that he made over the winter, and then suddenly fell 2m down into a well. Luckily the well still have snow inside it. If not it would be a 5m fall.
He got out with only some bruises and no major injuries.