Mongolian spots (technically named congenital dermal melanocytosis) are flat bluish-green, gray or light brown areas where cells called melanocytes have been trapped in a layer of the skin called the dermis.

CNN: What are Mongolian spots?

But in recent years it has become the home of huge empty lots and darkened projects, including Las Vegas Plaza and Fontainebleau Las Vegas, a hulking bluish-green tower that was 70 percent finished when construction ended.

WSJ: Asian firm buys site of stalled Vegas Strip casino

The north Strip is also home to the Fontainebleau Las Vegas, a hulking bluish-green tower that was 70 percent completed when the construction workers were dismissed, and the Sahara hotel-casino, which went dark in 2011, and only recently began renovations.

NPR: Stalled Las Vegas Strip Project To Get New Owner