Complex sentence: In 1957 Denver's original airport, Stapleton International Airport, was the eighth busiest airport in the nation and by the mid- 1980s it had become the seventh-largest airport in the world and fourth-busiest in the United States.
Split into simpler sentences: In 1957 Denver's original airport, Stapleton International Airport, was the eighth busiest in the nation. By the mid- 1980s it had become the seventh largest airport in the world and fourth busiest in the United States.