Ewens Ponds is a series of three water-filled limestone sinkholes on Eight Mile Creek 25 kilometres South of Mt. Gambier.
Each pond is a basin-shaped limestone dolineapproximately 9 metres (30 ft) deep and connected to the others by shallow watercourses called "races". The beds are covered with a fine silt layer and the floor of the third pond also contains a natural shallow cave. The ponds are located in a narrow band of native bush land, surrounded by cleared terrain. The landscape is characteristic of karst topography, shaped by the gradual dissolution of soluble limestone to form hollows and small caves, along with numerous large and relatively deep sinkholes (true cenotes).
The ponds contain extremely clear, high quality freshwater in which snorkellers and scuba divers can enjoy the wonder of swimming in a giant 'underwater garden', where the prolific plant life can easily be seen on the far side of each pond, more than 80 metres (260 ft) away in some areas. The clarity of the water also allows sufficient sunlight to penetrate that plant growth on the pond beds can reach up to 6 metres (20 ft) in height. The ponds are also occasionally affected by outbreaks of blue-green and other algae, though testing has found no evidence of health risks. In 2007 the South Australian Environmental Protection Agency suggested the algal blooms may be a result of continued concentrations of soluble nitrogen in both the ponds and the adjoining Eight Mile Creek, arising from infiltration of the groundwater by fertilisers, animal waste or wastewater.
The ponds are open to all open water certified scuba divers.