What is That Fenced in Area Behind Hughes Stadium in West Fort Collins?

I really want to know what this is, and I really have no idea!  It is a fenced in area just north of Hughes Stadium (The place where the CSU Rams play half of their games), right up against the Foothills.

My wife and I used the trail behind the stadium to hike up to Horsetooth Reservoir last Saturday to watch the USA Pro Challenge bicycle race.  It sits right off the trail that goes from the Stadium to the Reservoir. You can't miss it.  There is some sort of flat structure, but what is inside? There is a sign on the fence that reads:

RESTRICTED AREA
This area under 24 hour
video surveillance.

In Emergency - - call 911

This got our minds going crazy! What could it be?  An underground bomb shelter?  A top-secret communications center?... Or is it something as simple as a water treatment plant or some kind of sewer?  Do you know what it is?  Please tell me in the comment section below or at least take a good guess if you don't know.