Popular beaches around Sydney, Australia had to be closed on Tuesday after a huge algae bloom turned the water deep red.
Experts say the thick red algae is a natural phenomenon, sometimes caused by adverse weather conditions.
The blooms, which are often referred to as "red tides" or "sea sparkle," are more common during the spring and autumn months when the water temperature is warmer and there are greater movements in ocean currents.
Although it has no toxic effects, people are advised to avoid swimming in areas with discoloured water as the algae can be high in ammonia and can cause skin irritation.
With files from CNN