In some cities there's a whole world hidden from view. Like the secret passages of an ant colony, urban skyway or tunnel systems are bustling hives of activity where professionals, hospital workers, tourists or college students scoot from building to building below grade or one level above the street. These webs of life are an often-overlooked non-traditional real estate opportunity worth your attention.

The year is off to a brutal start for store closures. So far, there are more than 4,800 stores set to close. That's according to Coresight Research in a weekly tally of openings and closings that have been announced by various companies. There are also more than 2,200 openings announced, but the gulf of 2,600 closures is certainly concerning.