Undines are water nymphs. They are elementals; mythological creatures. They are very beautiful and (like all nymphs) immortal. One of the only threats to a nymph's eternal happiness is if she falls in love with a mortal and bears his child. Then she will lose her gift of everlasting life. Undines were said to be able to gain a soul by marrying a human and bearing his child, but then begins to age. They also appear in European folklore as fairy-like creatures. The name is sometimes used interchangeably with those of other water spirits.