Fossil Cave (5L81), formerly known as The Green Waterhole, is a cave in the Limestone Coast region of south-eastern South Australia. It is located in the gazetted locality of Tantanoola about 22 kilometres (14 miles) north-west of the city of Mount Gambier, only a few metres from the Princes Highway (Route B1) between Mount Gambier and Millicent. It is popular with cave divers and is notable for being both a unique paleontological site and the "type locality" for very rare crustaceans (syncarids - Koonunga sp.) which to date have been found only in caves and Blue Lake in the Mount Gambier region.