The Sisters are a geological rarity, even in such prominent cave-country as the Lower South East of South Australia. They appear as two intersecting, oblong-shaped "cenote"-style waterfilled sinkholes, but they were most likely formed from two "end collapses" of a single large cavern.  Each lake is about 40 metres across, and from the air the holes appear to be almost mirror-images of each other, being separated by a dry strip or saddle of limestone about 15 metres wide and 3 metres high.  The walls of both lakes are almost vertical faces which drop about 9 metres to the water, and they are "decorated" with many unusual keyhole-shaped solution holes and bulbous 'stromatolitic' growths (refer to 5L8).  Access to the saddle is awkward (but at least, possible!) for heavily-equipped cave divers, thanks to a primitive ramp-like cutting which was made sometime around the beginning of the 20th Century, but divers then need to tackle the dense cutting grasses and stinging nettles (as well as occasional bees and snakes) which surround the water before they can "get wet"!

Underwater, the features prove to be fairly simple mud-floored pools with small overhangs and maximum depths of about 20 metres or so, but the visibility in both holes is usually extremely poor during most of the year.  There is also a lot of rubbish in the form of car bodies, rolls of wire and other such items on the bottom, and the southern hole in particular has been heavily abused in this way over the years.

These sinkholes were reportedly discovered on May 5th, 1844 by Governor George Grey, who named them "Double Well".  The first known logged dives here occurred in November 1962 and January 1963, when local diver Philip "Mick" Potter found poor conditions similar to those so often encountered today.  However, his dive in the southern hole was interesting because he located a stolen Chevrolet, from which he retrieved its registration disk.  It is also rumoured that someone once found some sticks of gelignite wrapped in barbed wire in this feature, so divers should not play with anything they cannot identify ... just to be on the safe side!