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.
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."
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. "