The Louisiana Superdome in New Orleans was completed in 1975 and was dubbed the “Ninth Wonder of the World.” There are the Seven Wonders of the Ancient World, and the Astrodome in Houston, Texas, was opened in 1965 and was called the “Eighth Wonder of the World.”
 
The Superdome was being billed as the “Ninth Wonder of the World” in a newspaper story in January 1969—over six years before the building would open.
     
 
Wikipedia: Mercedes-Benz Superdome
The Mercedes-Benz Superdome, often referred to simply as the Superdome, is a domed sports and exhibition venue located in the Central Business District of New Orleans, Louisiana, United States. It primarily serves as the home venue for the New Orleans Saints of the National Football League (NFL), and is also the home stadium for the Sugar Bowl and New Orleans Bowl in college football. Plans were drawn up in 1967 by the New Orleans modernist architectural firm of Curtis and Davis and the building opened as the Louisiana Superdome in 1975. Its steel frame covers a 13-acre (5.3 ha) expanse and the 273-foot (83 m) dome is made of a lamellar multi-ringed frame and has a diameter of 680 feet (210 m), making it the largest fixed domed structure in the world. It is adjacent to the Smoothie King Center.
 
Wikipedia: Seven Wonders of the Ancient World
The Seven Wonders of the World or the Seven Wonders of the Ancient World is a list of remarkable constructions of classical antiquity given by various authors in guidebooks or poems popular among ancient Hellenic tourists. Although the list, in its current form, did not stabilise until the Renaissance, the first such lists of seven wonders date from the 1st-2nd century BC.
     
Newspapers.com
23 January 1969, Mamou (LA) Acadian Press, “Astrodome-Superdome Affair” by Randy Vidrine, pg. 5, col. 4:
If built, the Superdome will supposedly outmatch the Astro, in that many Super features will be added in order to make the New Orleans structure “the ninth wonder of the world.”
 
Newspapers.com
23 January 1969, The Gazette (Ville Platte, LA), “Comme Je Vois Les Sports” by Randy Vidrine, sec. 1, pg. 4, col. 6:
You see, Old Lou may get the last laugh—if and ever they build a dome in our Crescent City of New Orleans. “The ninth wonder of the world”—the Superdome—is described by Dave Dion, executive director of the stadium commission, as “a magnificently structure work, a 22nd Century Taj Mahal, and an architectural gem that will become one of the most famous buildings in the world.”
 
27 December 1969, State-Times (Baton Rouge, LA), pg. 8-A, col. 1:
Superdome Backers See 9th Wonder of World
By MACK SISK
NEW ORLEANS (UPI)—As the 1960s ended, sponsors of Louisiana superdome were promising that three years hence a man could stand in downtown New Orleans and gaze up at the ninth wonder of the world.
   
23 December 1972, The Louisiana Weekly (New Orleans, LA), “Time Out” by Jim Hall, sec. 2, pg. 7, col. 4:
The Louisiana Superdome is beginning to take form downtown. When completed, the dome is going to house all of the major sports attractions, and there will be plenty of jobs in the Ninth Wonder of the World for blacks.
     
17 January 1975, Dayton (OH) Daily News, “Si-ings: You Could Put the 8th Wonder Inside the World’s Ninth Wonder” by Si Burick, pg. 18, col. 1:
Well, I saw it; the ninth Wonder of the World. If the Astrodome in Houston is still being billed as the eighth Wonder, then why not refer to the New Orleans Superdome as the ninth?
   
16 March 1975, The Sunday Courier and Press (Evansville, IN), “Dominion ponders jump” by Pete Swanson, pg. 12C, col. 4:
“You take America’s most interesting city and the ninth wonder of the world…. If you were a basketball player, wouldn’t you like to play in the Superdome? Wouldn’t you like a two-day vacation in New Orleans?”
(New Orleans basketball coach Ron Greene.—ed.)
 
Fansided—Who Dat Dish
Should New Orleans replace the Mercedes-Benz Superdome?
by Kervin Aucoin May 31, 2016
Nestled near the Mississippi river and completed in 1975 at a cost of $134 million, the Superdome became a modern marvel. A July 6, 1975 article in the Chicago Tribune, by way of the Associated Press, claimed it had been called “The ninth wonder of the world.” When plans were being drawn up in the 1960s, then governor of Louisiana John McKeithen exclaimed the Superdome would be “the greatest building in the history of mankind!”