Mount Gambier is a town built on holes - a volcanic and limestone landscape riddled with sinkholes and flooded caverns - and Engelbrecht Cave runs right beneath its streets. Guided tours descend into the dry upper chambers, where the story of the cave's past life (it was once used to dump distillery waste, and later cleaned out) is as strange as the formations. Below, crystal-clear water fills passages that are among the best-known cave-diving sites in the country. It is an easy, atmospheric introduction to the hidden underworld of the Limestone Coast.