 Mysterious metal monoliths have appeared around the world in the last few years, including one a few weeks ago in Las Vegas. The most recent one to pop up is in northern Colorado near Howling Cow Cafe. One of the customers came in looking for it. Where is the alien structure? I don't know. What are you talking about? Coming up on All Things Considered, a look at the perplexing metal structures. That story just ahead on KNPR and KNPR.org.