“Earth Hour’s unique display of darkness has become a global phenomenon.” (http://www.wwf.org.uk/about_wwf/press_centre/scottish_press_centre/?7522/Giant-Kelpies-to-go-dark-for-Earth-Hour)
“The Northern Lights (Aurora Borealis) and the Southern Lights (Aurora Australis) are amongst nature’s most spectacular displays. The British Antarctic Survey, who will be switching off for Earth Hour…” (http://earthhour.wwf.org.uk/10-facts-about-the-aurora-lights/#.VRaDHGIgGSM)
Can you display darkness?
If you can switch off the aurora you can pretty much do what you like.