At 8:30 p.m. local time on Saturday, cities across the world are flipping off the lights.
They’re some of the world’s most famous buildings, but Saturday night, they blended into the darkness. From the Sydney Opera House to the Colosseum to the Vegas Strip, landmarks on every continent took part in Earth Hour, turning off all lights for an hour to draw attention to the need for climate change.
(More on TIME.com: Can Earth Hour galvanize the global warming fight?)
2011 marks the fifth year of cutting power for the hour in the initiative organized by the World Wildlife Fund. Organizers estimate over 1 billion people take part in the global blackout. Cities have embraced the moment to see their famed structures stand in darkness, and local utilities companies note a sharp decrease in usage.
“Last year during Earth Hour … we were able to save enough energy to remove more than 124,000 pounds of carbon dioxide from the atmosphere. That’s the equivalent of planting 15 acres of trees,” a Chicago utilities spokeswoman said.
And aside from the environmental benefit, it’s a view of the worldwide monuments that we rarely get a chance to see.
(More on TIME.com: See video of Thomas Edison and the lightbulb)