A missile silo is not something I'd expect to see in Colorado. A missile silo is in Greeley and I think this qualifies as a weird landmark.

There are a few weird landmarks around Colorado like the hotel from The Shining and a UFO Watchtower. I didn't even know what a missile silo was until I saw this one in Greeley. Greeley is in Weld County, which has had four nuclear missiles in underground silos from 1961-1965. These were around until better missiles came along.

Free tours are held here and there's a campground too. I bet you've never seen a silo and if you have, I bet you've never seen this much concrete before. There's enough concrete here to build a four-foot wide sidewalk from Greeley to Salt Lake City.

Sources: Denver7, Weldgov and Roadtrippers