Scores of photographers have been drawn to Yosemite National Park to see a 'firefall'.
The phenomenon appears to be a cascade of lava falling from east of the the El Capitan rock formation.
However, it is actually an optical illusion caused by the light of setting sun falling on Horsetail Fall waterfall.
It typically happens for a few weeks in February each year, though can occur at other times.
The firefall is dependent on favourable conditions, such as sparse cloud cover and a flowing waterfall.
Social media users have been sharing their photos of the event.
Many photographers spent a long time preparing for the event:
This is what it looks like the rest of the time:
The firefall is named after a made-made fire fall which used to take place in Yosemite. The owners of Glacier Point Hotel, at Glacier Point, used to spill hot embers down the mountainside to create a viewing spectacle. However, the practice was discontinued in 1968.
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