Mile High City, the Capitol in Denver.

Why is Denver called the Mile High City?

Sources:
https://www.atlasobscura.com/places/mile-high-steps-at-the-colorado-state-capitol
https://mtstandard.com/news/state-and-regional/denver-taking-steps-to-reassess-mile-high-status/article_e6393261-db86-5e31-868b-387ea988e4c6.html
https://www.5280.com/2015/05/only-in-colorado-the-capitols-mile-high-steps/
http://skyrunner.com/story/coheights.htm
https://www.nytimes.com/2003/10/05/us/denver-journal-hoping-third-try-is-right-mile-high-city-marks-the-spot.html