Surf's up! But wait, what's that red stuff?
No, it's not ketchup and Jaws hasn't attacked - it's just algae.
The marine plant managed to close Australia's world-famous Bondi Beach, as well as Sydney's Clovelly Beach.
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A swimmer stops short of a red algae bloom at Sydney's Clovelly Beach
If you want to cool off, the algae, known as Noctiluca scintillans or sea sparkle isn't toxic - but it can irritate the skin.
Local lifeguard Bruce Hopkins said it made the water look like "tomato sauce."
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The red algae, known as Noctiluca scintillans or sea sparkle, has no toxic effects
Irene Eristian, 33, told News.com.au the water colour was "intense." "I wasn't sure if I should let my daughter into the water as I wasn't sure what it was."
This isn't the first time water has turned blood-red this year. In August a lake in the South of France made the headlines after changing colour, but experts said the phenomenon was "completely natural".
Patricia Estebe from Camargue's tourist office explained: "This phenomenon is a result of the salt content in brine shrimp Artemia salina and algae Dunaliella salina. When the salt concentration is very high - which is the case before harvest - the brine shrimp die and saline algae proliferates giving this unusual colour. "