Yes, the egg will probably explode leaving a nasty smell and mess. Mother swans do not abandon viable eggs, but will kick non fertile or bad eggs from the nest. Since you do not know how old the egg is or the circumstances, such as if the nest was flooded and the egg was in the water for some time, etc., there is no way to tell if it could produce a viable healthy cygnet. Were there any signs of a nest or adult swans in the area? If so, you might try to put the egg back in the nest without disturbing the parents or you getting attacked. If the egg is no good, the mother swan will get rid of it. If there are no swans or nest in the area, we would suggest you put it back where you found it so that if it does explode, you will not incur the mess. The Regal Swan