When Gulliver is in Luggnugg, he is "asked by a Person of Quality" whether or not he has ever seen one of their most peculiar kinds of residents called Struldbrugs, or Immortals in English. He is informed that such immortals exist here, and they are born with a solid red circle in the middle of their foreheads, immediately atop the left eyebrow, and this is how the person is identified as one who is going to live forever. The spot changes colors as the immortal ages: turning green, then blue, and then deep black.
Gulliver is amazed by this strange fact, and he proceeds to elaborate on how wonderful it would be to live forever as well as all the things he would do if he were so lucky to be a Struldbrug. However, his interpreter finally informs him that his conjectures are way off because Gulliver is assuming eternal youth, and this is not the case with the Struldbrugs. Instead, they age and age and age, eventually becoming unable to understand language or be understood, and they are miserable decrepit drains on the economy in the end.