A Nickname Measured in Miles
The U.S. city known as the "Mile High City" is Denver, the capital of Colorado. The nickname is wonderfully literal: the city sits at an elevation of exactly one mile - 5,280 feet - above sea level.
The Step That Proves It
The mile-high claim is marked in a memorable way. On the steps of the Colorado State Capitol building in Denver, there is a spot that sits precisely one mile above sea level. Visitors often stop to stand on it. Because surveys over the years have refined the exact measurement, more than one step has been marked over time, but the idea remains the same: you can literally stand one mile high in the heart of the city.
Life Above the Plains
Denver sits where the flat Great Plains meet the foothills of the Rocky Mountains. Its high elevation gives the city thin, clear air and plenty of sunshine. The altitude is a point of local pride, woven into the names of sports teams and local culture. The thinner air can even surprise visitors, who sometimes feel the effects of the elevation until they adjust.
Source
This article was written using information from Wikipedia.