Today, the vacancy rate is about 28%--the highest vacancy rate of any submarket in the area--thanks to about 1.5 million sf of subleased on the market, or about to come on to the market, says Chris Phenicie, a broker and co-owner of Commercial Colorado.

Just a few years ago, there wasn't even 1.5 million sf in the entire market, much less subleased space, he says.

Level 3, the struggling telecommunications giant caught in a stock slump, will be subleasing about 300,000 sf it has under construction, says Phenicie and other brokers. Level 3, which recently announced plans for layoffs and is headquartered in an 850,000-sf campus along the corridor, says it is continually monitoring its real estate holdings in Colorado and around the world, but won't address specific properties.