[Related: 10 simple tips to be green without trying too hard]
"Many environmentalists see humans as fundamentally destructive and want to force individuals to conserve," Michelle Minton, who came up with HAH in 2009, told MSNBC.com. "Earth Hour is an attempt to convince lawmakers that the majority of the population wants them to clamp down on progress."
Though about 235 people have signed on to keeping the lights on, the World Wildlife Fund says that 135 countries -- and the International Space Station -- have pledged to turn theirs off on Saturday.
Here, the Las Vegas Strip is lit up before Earth Hour from the House of Blues Foundation Room inside the Mandalay Bay Resort & Casino in 2011.
On Saturday at 8:30 p.m., cities around the world will mark Earth Hour by leaving their landmarks in darkness. Launched in 2007 by the World Wildlife Fund, it's intended to be a call to action on climate change and clean energy. Here's a look at some Earth Hour images from years past.