Mount Gambier's Blue Lake is renowned for its striking blue color, attracting visitors from around the world. Swiss tourist Kathrin Ottiger remarked, "It's blue as the name says," while visiting a lookout this week. However, the reasons behind the lake's vibrant hue are more complex than many might think.

Local residents offered various theories about the lake's color. One girl suggested, "Because people clean it," while a man speculated, "I think it's to do with the chlorine from the pump station." Another joked, "Because there's dye in the water!" None of these explanations are accurate.

The Blue Lake's unique color results from two geological phenomena in South Australia's southeast. Beneath the surface lies a layer of limestone formed millions of years ago from the remains of sea creatures. Additionally, volcanic activity in the region, including an eruption from Mount Gambier about 6,000 years ago, contributed to the lake's formation. This eruption created four craters, with the Blue Lake being the largest.

The lake's color changes throughout the year, becoming a brilliant cobalt blue in late November. Hydrogeologist Ian Lewis explained, "From about now in the spring and through summer, the top part of the lake warms up — about the top 10 metres. As the water gets warmer, it starts to precipitate tiny little micro-crystals that reflect the blue light from the sun. They absorb the red light, but we see the blue light."

This vibrant blue persists until the end of February, when the water begins to cool. Lewis noted that scientists have studied the lake's mixing process, where the warmer water at the top cools and sinks, allowing the cooler water from the bottom to rise, resulting in a complete turnover during winter.

In contrast, the nearby Valley Lake, which is less known, is used for water sports and has a green color due to its shallowness. Lewis explained, "The Blue Lake is 70 metres deep; the Valley Lake is no more than probably about 15 metres deep. You can get an algal bloom in the Valley Lake very easily, and it stays quite a bit warmer through these summer months."

The Blue Lake serves as a reservoir for Mount Gambier's drinking water. Simon Sherriff, regional water security manager, stated, "It's very unique — Blue Lake/Warwar is the only blue volcanic lake in Australia and, to our knowledge, the only one in the world that is currently used for drinking water supply." The limestone in the lake, known as calcium carbonate, contributes to the hardness of the tap water, which can feel less soapy when washing hands.

The Blue Lake, referred to as Warwar in the Indigenous Boandik language, means "home of the crow." Elders Uncle Ken Jones and Aunty Michelle Jacquelin-Furr noted that there is no Dreamtime story related to the lake's color. Aunty Michelle reflected, "It's always a special thing to drive past, look at it and know that this has been here for thousands and thousands of years."

Visitors can learn more about the lake's depth, which is 70 meters, and its circumference of 3.5 kilometers. Swimming is not permitted, but the Apex Lookout offers the best views. Amanda Stevens, the city’s visitor economy coordinator, encourages people to visit the lake year-round, noting its beauty in winter as well.

For those looking for souvenirs, local businesses offer products like "Blue Lake Water" and Blue Lake Lemonade, which are popular among residents and tourists alike. Marketing manager Ebony Bennett highlighted the significance of the lake, stating, "The blue is definitely the most popular out of the whole four flavors and obviously it has got a strong history with Mount Gambier."