Question: Where is the cave that was first documented in the oldest continuously published monthly magazine in the United States?

Context paragraphs:
The Harvard Advocate: The Harvard Advocate, the art and literary magazine of Harvard College, is the oldest continuously published college art and literary magazine in the United States.  The magazine (published then in newspaper format) was founded by Charles S. Gage and William G. Peckham in 1866 and, except for a hiatus during the last years of World War II, has published continuously since then.  In 1916, "The New York Times" published a commemoration of the "Advocate"'s fiftieth anniversary.  Fifty years after that, Donald Hall wrote in "The New York Times Book Review" that "In the world of the college – where every generation is born, grows old and dies in four years – it is rare for an institution to survive a decade, much less a century.  Yet the Harvard Advocate, the venerable undergraduate literary magazine, celebrated its centennial this month."  Its current offices are a two-story wood-frame house at 21 South Street, near Harvard Square and the University campus.

Gypsum Cave: Gypsum Cave is a cave located east of Las Vegas.  It was first documented by Mark Raymond Harrington in a 1930 edition of "Scientific American".  Up until about 11,000 BC (11,000 radiocarbon years ago), Gypsum Cave was inhabited by the Shasta ground sloth.

Scientific American: Scientific American (informally abbreviated SciAm) is an American popular science magazine.  Many famous scientists, including Albert Einstein, have contributed articles in the past 170 years.  It is the oldest continuously published monthly magazine in the United States(though it only became monthly in 1921).

Harper's Magazine: Harper's Magazine (also called Harper's) is a monthly magazine of literature, politics, culture, finance, and the arts.  Launched in June 1850, it is the second-oldest continuously published monthly magazine in the U.S. ("Scientific American" is the oldest,but did not become monthly until 1921).  The current editor is James Marcus, who replaced Christopher Cox in March 2016.  "Harper's Magazine" has won twenty National Magazine Awards.

Proceedings (magazine): Proceedings is a 96-page monthly magazine published by the United States Naval Institute.  Launched in 1874, it is one of the oldest continuously published magazines in the United States.  "Proceedings" covers topics concerning global security and includes articles from military professionals and civilian experts, historical essays, book reviews, full-color photography, and reader commentary.  Roughly a third are written by active-duty personnel, a third by retired military, and a third by civilians.  "Proceedings" also frequently carries feature articles by Secretaries of Defense, Secretaries of the Navy, Chairmen of the Joint Chiefs of Staff, and top leaders of the Navy, Marine Corps and Coast Guard.

The Friend (Quaker magazine): The Friend is a weekly Quaker magazine published in London, UK.  It is the only Quaker weekly in the world, and has been published continuously since 1843.  It began as a monthly and in January 1892 became a weekly.  It is one of the oldest continuously published publications in the world still in operation.  Others (e.g. "Punch") which began publication before "The Friend" have had lengthy interruptions in publication and/or have closed down.

Flying Models: Flying Models was a monthly magazine dedicated to model aviation published by Carstens Publications.  It was the oldest continuously published magazine dedicated to model airplanes, having started as "Flying Aces" in October 1928.  "Flying Models" was acquired by Carstens Publications in 1969, and ceased publication in 2014.  The headquarters of the magazine was in Newton, New Jersey.

Monthly Review: The Monthly Review, established in 1949, is an independent socialist magazine published monthly in New York City.  The publication remains the longest continuously published socialist magazine in the United States.  The journal has an impact factor of 0.460.

Angelus (magazine): Angelus is a weekly magazine published jointly by The Tidings Corporation and the Roman Catholic Archdiocese of Los Angeles, the most populous Catholic archdiocese in the United States.  The magazine began publication in 1895 as a newspaper named "The Tidings" and is the oldest continuously published Catholic periodical on the west coast of the United States.  It is also the oldest weekly periodical in the Los Angeles market.  The last issue of "The Tidings" was published in June 2016; in July 2016 it was transformed into the multimedia news platform "ANGELUS" (AKA "Angelus News").

Baltimore (magazine): Baltimore is a monthly magazine published in Baltimore, Maryland by Rosebud Entertainment L.L.C., a company owned by Steve Geppi.  It is the oldest continuously published city magazine in the continental U.S. and was first printed in 1907 by the Baltimore Chamber of Commerce.  In 1977, Philip Merrill's Capital-Gazette Communications purchased "Baltimore" from the Chamber; Merrill sold the magazine to a group of investors in 1992.  Steve Geppi acquired "Baltimore" in 1994.  It is a member of the City and Regional Magazine Association (CRMA).

Supporting paragraphs: Gypsum Cave Scientific American
Answer: east of Las Vegas