On Saturday evening, skylines across the world will go dark as millions of people turn off their lights. But why the blackout and what's it all about?
What is it?
Every year thousands of landmarks across the globe switch off their lights in the world's biggest annual environmental event.
Called Earth Hour, it's organised by conservation organisation WWF. Last year, millions of people across 178 countries supported the action as a sign that they care about the future of the environment.
Last year, more than 10.4 million people in Britain joined in with the Palace of Westminster, Buckingham Palace, The Shard, The Ritz hotel , Old Trafford and Edinburgh Castle among the landmarks which dimmed their lights.
Across the world more than 350 landmarks from New York to Beijing disappeared from view between 8:30pm and 9:30pm local time, including the Eiffel Tower, the Empire State Building and Tokyo Tower.
When is it?
Earth Hour 2017 takes place at 8:30pm GMT on Saturday March 25. This year teams around the world will be using the movement to shine a light on the climate issue most relevant in their country or region.
In Brazil, people will be asked to support protection of one of the country’s many biodiversity hotspots from climate change, while citizens in South Africa will focus on renewable energy. In China, WWF is working with businesses to encourage a shift toward sustainable lifestyles.
When did it start?
This year marks the tenth anniversary of Earth Hour which started back as a symbolic event in 2007 in Sydney, Australia. The UK got involved a year later and it has since spread around the world.
Siddarth Das, executive director at Earth Hour Global, said: "We started Earth Hour in 2007 to show leaders that climate change was an issue people cared about.
"For that symbolic moment to turn into the global movement it is today, is really humbling and speaks volumes about the powerful role of people in issues that affect their lives."