Alien base found in Colorado?

On January 28, 2018, a witness flight over Colorado. Here, he discovered a large unidentified structure on the floor.

He caught pictures and it is hard to say what is it. Is an alien base? A military secret base?

Eyewitness statement:

I took these pictures from my flight over Colorado last week. This appears to be a massive structure isolated in the mountains with no visible roads leading to or from.

Source, 08th February 2018