Explore South Australia Adelaide The Barossa Valley The Fleurieu Peninsula Kangaroo Island The Southeast The Murray River The mid-north Port Augusta The Eyre Peninsula The Nullarbor Plain Coober Pedy Flinders Ranges National Park The Strzelecki Track Lake Eyre The Birdsville Track The Oodnadatta Track Share Set close to the border with Victoria, MOUNT GAMBIER is the southeast’s commercial centre and South Australia’s second most populous city. The city sprawls up the slopes of an extinct volcano whose three craters – each with its own lake surrounded by heavily wooded slopes and filled from underground waterways – are perfect for subterranean diving. The Blue Lake is the largest and most stunning of the three, up to 70m deep and 5km in circumference. From November to March it’s a mesmerizing cobalt blue, reverting to a moody steel blue in the colder months. There are lookout spots and a scenic drive around the lake. The second-largest crater holds Valley Lake and a Wildlife Park, where indigenous animals range free amid native flora; there are also lookouts, walking trails and boardwalks. West of the city centre is the extensive complex of underground caverns at Engelbrecht Cave. The centrepiece of Mount Gambier itself is Cave Gardens, a shady park surrounding a deep limestone cavern with steps leading some way down; the stream running into it eventually filters into the Blue Lake. East of the city is Umpherston Sinkhole, also known as the Sunken Garden for its Victorian-era terraced gardens – they are floodlit at night when possums come out to feed.