Global landmarks from Sydney's Opera House to the Forbidden City, to the glittering Las Vegas Strip, will be plunged into darkness Saturday as activists bid to reinvigorate the climate change fight.
Hundreds of millions of homes, in scores of cities scattered around 125 countries will also join the great Earth Hour switch-off, which comes just months after disappointing UN climate talks in Copenhagen.
Iconic structures like the Empire State Building and Egypt's pyramids will heed conservation group WWF's call to turn off the lights in the name of environmentalism, creating a wave of darkness that will roll across the world.
United Nations secretary-general Ban Ki-moon called the campaign, now in its fourth year and expected to attract record participation, "both a warning and a beacon of hope".
"Climate change is a concern for each of us. Solutions are within our grasp and are ready to be implemented by individuals, communities, businesses and governments around the globe," Ban said.
Sydney's Opera House and Harbour Bridge will informally kick off proceedings when they go dark at 8:30 pm (0930 GMT) along with millions of Australian households.
The pattern will be replicated at the same local time across the world, spreading across Asia to the Middle East, Europe, Africa and the Americas and including 1,200 famous landmarks.
Beijing's Forbidden City and Bird's Nest Stadium are among the participants along with dozens of cities in China, the world's biggest carbon polluter, where giant panda Mei Lan is an Earth Hour ambassador.
Hong Kong's renowned neon waterfront will temporarily dim as will the Singapore Flyer observation wheel and office buildings in Jakarta and Seoul, before India's massive urban centres of Delhi and Mumbai join the power-down.
London's Big Ben and Manchester United's Old Trafford football ground are set to take part amongst Europe's best known spots including Paris's Notre Dame cathedral and the Trevi Fountain in Rome.
In America, some 30 states are on board with Mount Rushmore, San Francisco's Golden Gate Bridge and Chicago's 110-storey Sears Tower all due to go dark.
But in Bangkok, city authorities are under military orders to halt their Earth Hour campaign for security reasons, as anti-government protesters plan another major rally.
Earth Hour started in 2007 in Sydney and now enjoys widespread support both from the public and big business, including Google, Coca-Cola and McDonald's.
This year, even users of ubiquitous Twitter and Facebook are able to show their support with special applications that turn their displays dark.
In December, two weeks of talks in Copenhagen failed to produce a binding commitment to limit global warming or set out concrete plans for doing so.
Explore further: Climate change poses growing health threat, UN reports