About 40 kilometers north-east of Canberra, in Australia, right next to the Federal Highway is a large lake, but you might not always see it. Depending on which time of the year or day you visit, there might be a large expanse of water or a swampy pastureland.

Lake George is extremely unpredictable. Its waters come and go like a mirage. There are stories of the lake’s waters retreating up to a kilometer from the shore in the course of a single night. When full, the lake spreads over 155 square kilometers with its eastern end lapping against the side of the highway. But often it dries to such an extent that the lake-bed is used for grazing.

The almost dry bed of Lake George. Photo credit: Jerry Skinner/Flickr

Lake George is one of the world's oldest lakes, believed to have formed more than a million years ago. Originally, there was no lake and small streams from the Great Dividing Range drained to the Yass River. But then there was a geological uplift and the Lake George Escarpment rose up creating a natural dam that blocked the creeks from reaching the river, and the lake was formed. Because Lake George has no outlet, it has accumulated all the salts and nutrients that have flowed in from its catchment over the millennia. Consequently, when wet, the lake’s waters are as salty as that of the sea.

In the early 1800s, the lake was significantly larger—large enough to support a commercial Murray cod fishery, but by 1840s it was so dry that one could drive across the middle. When water began to rise again a few decades later, it inspired ideas of a lakeside resort complete with paddle-steamers. But by the turn of the next century, the lake had dried again and neglected boathouses, jetties, decaying boats and launches lay on the lake’s former shore.

Photo credit: Andrew Jens/Google Maps

Australian geologist Patrick De Deckker said the last time he saw the lake full was in 1971. A decade earlier, there was a thriving fishing industry in Lake George with an enormous population of redfin. But in the late 1960s, Lake George almost dried up and the fish population crashed. In 1986 the lake was dry again, then wet in 1996, then dried out completely from 2002 to 2010. Since then, the lake has started to fill, and on September 2016, the level of water in the lake was reported to be high.

What causes Lake George’s water level to fluctuate so dramatically was a mystery for a long time. Some people believed that the water came from a secret underground spring and drained away through a crack in

the earth, to China or New Zealand or even Peru.

Photo credit: Nicholas Cull/Flickr

But Patrick De Deckker explains, “Lake George is actually a depression that turns into a lake when it fills. There's always water below the lake floor, and amazingly, it is saline, but if you have more rainfall, the lake fills up.”

Lake George is fed entirely by precipitation and runoffs, and the only way water can leave the lake is by evaporation. Because the lake is so shallow, the effect of each of these natural processes becomes more noticeable than in deeper bodies of water. Besides, there is a tendency for strong winds to blow the water from one side of the lake to the other—an effect similar to a storm surge—which explains the mysterious filling and drying episodes that have been observed to occur in the span of a few hours.

When there is water, Lake George becomes an important breeding ground and refugee habitat for water fowls, mammals, reptiles and amphibian. Over two hundred species of animals and birds have been observed here.

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Photo credit: Ian Sanderson/Flickr