Mount Gambier 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, and guided tours are offered by Aquifer Tours (daily on the hour: Feb– May & Sept– Oct 9am–2pm; June– Aug 9am– noon; Nov– Jan 9am–5pm, 45min; $7; Tel:08/8723 1199). The second-largest crater holds Valley Lake and a Wildlife Park (daily 7am– dusk; free).
The centrepiece of Mount Gambier itself is Cave Gardens, a shady park surrounding a deep limestone cavern with steps leading down; the stream running into it eventually filters into the Blue Lake.
The CDAA issues permits for snorkelling in the crystal-clear waters of Piccaninnie Ponds Conservation Park and Ewans Pond Conservation Park, both south of Mount Gambier near Port Macdonnell. At Piccaninnie Ponds, a deep chasm with white limestone walls contains clear water that is filtered underground from the Blue Lake. East of the city is Umpherston Sinkhole (open access), also known as the Sunken Garden for its Victorian-era terraced gardens – they are floodlit at night when possums come out to feed.