Most lakes look a little different from season to season. They freeze, they flood, they turn muddy after a storm. But one lake in Australia puts on a show so dramatic that people plan trips around it. Every year, almost on schedule, it changes color.
It's called Blue Lake, near the town of Mount Gambier in South Australia. Each November, it turns a deep, brilliant turquoise. Then, from late February into March, it gradually fades back to a duller blue. The exact cause is still debated, but the leading explanation is that summer warmth makes tiny crystals of calcium carbonate form in the water, and they scatter blue sunlight.
A Lake Inside a Volcano
Blue Lake sits in the crater of a dormant volcano. Experts disagree on when the volcano last erupted, with estimates ranging from about 4,300 years ago to 28,000 years ago. If the youngest date is right, it could be the most recent eruption on the Australian mainland.
The crater once held four lakes. Two of them dried up as the water table dropped, and one is now a picnic spot. Blue Lake remains. It's about two-thirds of a mile long, well under half a mile wide, and has a fairly flat bottom with an average depth of about 238 feet. It also supplies drinking water for the town of Mount Gambier.
How the Color Change Works
Remember that November is late spring in Australia, with summer right around the corner. As the weather warms, the lake's surface layer heats up to around 68°F. Scientists think that warmth causes calcium carbonate, the same mineral found in limestone and chalk, to come out of the water and form microscopic crystals. Those crystals scatter the blue part of sunlight, giving the lake its striking glow.
In winter, the lake mixes from top to bottom. Recent research suggests it becomes somewhat murkier then, as tannins and calcium carbonate particles get spread through the water. The angle of the Sun matters too, changing how the color looks to our eyes. Even tiny floating organisms moving up and down may play a part.
Divers Found Surprises Below
Researchers have explored the lake's depths. A 1967 survey found the deepest point at about 253 feet. In 1985, cave divers led by Peter Horne studied the water's temperature and clarity. They discovered a freshwater sponge species and a field of hollow rock formations called stromatolites, found mostly along one side of the lake down to about 130 feet.
Later dives, including one in 2008, collected samples from the lake bed, where the water temperature dropped to about 57°F.
A Place With Deep Roots
Long before European settlers arrived, the Boandik people lived in this area. In 2022, the local city gave the lakes dual names in their language. Blue Lake is now officially Blue Lake / Warwar, which means "crow country."
The lake has a famous legend, too. In July 1865, Australian poet Adam Lindsay Gordon is said to have jumped his horse over a fence onto a narrow ledge above the lake and back again. A memorial obelisk marks the spot.
Today, a road and walking track of about 2.2 miles loops around the rim. If you ever find yourself in South Australia in early summer, it's worth the trip. Few places let you watch nature flip a switch on such a grand scale.
