According to a theory advanced by Paracelsus, an Undine is a water nymph or water spirit, the elemental of water. They are usually found in forest pools and waterfalls. They have beautiful voices, which are sometimes heard over the sound of water. According to some legends, Undines cannot get a soul unless they marry a man and bear him a child. This aspect has led them to be a popular motif in romantic and tragic literature.