At 8:30 p.m. tonight (Saturday March 31), cities across the globe will voluntarily blackout for Earth Hour 2012. The Empire State Building, Sydney Opera House, Eiffel Tower and Great Wall of China will all go dark for one hour as a symbolic gesture of each nation's commitment to sustainability.

Details and the official video inside

The annual event known as Earth Hour is an open source idea - anyone can run it, and anyone can be a part of it. Perhaps that's why in just six years' time it's become the world's largest voluntary action for the environment, uniting communities around the world to celebrate a commitment to the planet by switching off lights for one hour on the last Saturday of March.