Firefall optical illusion lights up Yosemite National Park

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Yosemite Valley, Yosemite National Park, California, USA

It's not Lava - it's an optical illusion.

Every February, Horsetail Fall in Yosemite National Park lights up as if it were on fire, and people flock to the falls hoping to get a glimpse of the phenomena that only happens for 20 days in February around sunset.

It's an incredible illusion that can only happen under perfect circumstances: Clear skies, enough rain, and sun rays that hit just right.

See also: Yosemite as you've never seen it

Onlookers this year got a special treat.

A National Parks Service Spokesman told CNN: "The waterfall is bigger than it has been in a long time due to all the rain and snow we have received,"

Want to check out the firefalls? The park suggests heading to the El Capitan picnic area because in that area the waterfall is visible from the road.

If you're lucky enough to see it in person - make sure you have your camera ready to go because the phenomena only lasts about 10 minutes.

18 weird natural phenomena around the world

18 weird natural phenomena around the world