Camp served as a researcher at the Philadelphia Naval Yard along with his fellow writers Isaac Asimov and Robert A. Heinlein. De Camp eventually rose to the rank of lieutenant commander in the U.S. Navy as a reserve officer. De Camp was a member of the all-male literary and dining club the "Trap Door Spiders" in New York City, which served as the basis of Asimov's fictional group of mystery solvers the "Black Widowers." De Camp himself was the model for the character named "Geoffrey Avalon." De Camp was a founding member of the Swordsmen and Sorcerers' Guild of America (SAGA), a loosely knit group of Heroic fantasy writers that was founded during the 1960s and led by Lin Carter, with entry by credentials as a fantasy writer alone. The de Camps moved to Plano, Texas, in 1989, and Sprague de Camp died there on November 6, 2000, seven months after his wife, on what would have been her birthday, just three weeks before his own 93rd birthday. His ashes were inurned, together with hers, in the Arlington National Cemetery in Virginia. De Camp's personal library of about 1,200 books was acquired for auction by Half Price Books in 2005. The collection included books inscribed by fellow writers, such as Isaac Asimov and Carl Sagan, as well as de Camp himself. Coinage of "Extraterrestrial" and "E.T." "Extraterrestrial", a coinage from "extra" + "terrestrial", meaning from beyond earth, is attested as an adjective as early as 1868, according to the Oxford English Dictionary. Its first use in connection with life beyond earth was likely by H. G. Wells, in his 1898 novel The War of the Worlds. L. Sprague de Camp is credited with its first usage as a noun with the meaning of "alien life" and with coining the abbreviation "E.T." in the first part of his two-part article "Design for Life", published in the May 1939 issue of Astounding Science Fiction. Hypothesis regarding sea serpent sightings De Camp made a suggestion in 1968 that fossil discoveries had influenced modern reports of sea monster sightings. The suggestion was made in his article "Dinosaurs in Today's World" in The Magazine of Fantasy & Science Fiction v. 34, no. 3, March 1968, later reprinted in his nonfiction collection The Fringe of the Unknown, Buffalo, New York: Prometheus, 1983. In 2019 de Camp's theory was scientifically tested by C. G. M. Paxton and