Aussies didn't have to fear the end of the world, though, flocking to take photos of the natural wonder.
"We could see massive, slightly green clouds approaching and started to get excited as Sydney thunderstorms are a great light show," Daniel Povall told CNN.
"It was starting to get very windy but it was thrilling watching the Independence Day style cloud approach."
University of Sydney Associate Professor Alexandra Martiniuk captured the view near Manly Beach in suburban Sydney.
This feat of Mother Nature is known as a shelf cloud.
"Shelf clouds are generally associated with lines of strong thunderstorms," explained CNN meteorologist Taylor Ward. "They are low hanging clouds that represent the leading edge of a line of thunderstorms and are frequently associated with strong winds.
These menacing clouds are quite common in Sydney, although they can happen anywhere in the world.
After the line of storms, viewers are often treated to some spectacular sunsets. The contrast of the dark cloud over the cool blues of the ocean make for some breathtaking photos.
"The backdrop of the ocean, plus the flat landscape, plus a lot of people means lots of photos when they occur," Ward said.
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