Australia - Mt Gambier and the karsts of the Limestone coast.

There are 30 million years ago, well before the installation of any volcanic activity, Mt Gambier region was covered by a warm ans shallow ocean.
Diverse fossil, crustacean shells and fish bones fell to the ocean floor and  have formed over time a thick layer of limestone that forms the underbody of the region, which takes its name : the Limestone coast .

Karstification of this layer is made by dissolving the limestone by carbonic acid and underground drainage. The erosion has formed, in the basement, interconnected caves .
                                                    Karstic landscape type - doc. Geowiki

Engelbrecht cave forms a complex of limestone caves under Mt Gambier and the city of the same name. The exploration and cleaningof the caves now offer 600 meters of boardwalk accessible to all, with a succession of lakes and passages ... other rooms are a delight for divers who can swim under the city.

                          Mt Gambier - Engelbrecht cave - photo RAA Tourism

                Diving in a cave - photo Cave Divers Association of Australia - CDAA

              Mt Gambier - Engelbrecht cave - click on the picture takes you to the video / Vimeo

Cave whose roof collapsed and fell on the floor by creating terraces - sinkholes - found another function, after having served to potable water reserve to the first inhabitants of Mt Gambier .

In the center of the city, The Cave gardens , which have two holes and a drainage waterfall, were restored in 2001 in the spirit of the glorious 20s. At nightfall , a sound and light tells the stories of the Aborigines.

Earlier , another sinkhole , the Umpherston Sinkhole, named in honor of its owner, James Umpherston houses a beautiful garden .
He created in 1884 a Victorian garden, frequented by locals of Mt.Gambier who came cannoter on the small lake covering a third of the sinkhole .

Umpherston Sinkhole - photo archives / via blog adonline

After his death in 1900 , the garden almost disappear before being restored in 1976 by ​​the Department of Water Affairs and Forestry .

                        Mt Gambier - Umpherston Sinkhole - photo Statiflickr

                       Mt Gambier - Umpherston Sinkhole - photo loma2010.blogspot

Repopulated with Hydrangea macrophylla , three indigenous Dicksonia antarctica tree, and ivy cascading on its walls, the garden comes alive after dark , when the possums come to feed .

Sources :

- Global Volcanism Program - Newer Volcanics Province

- Office of minerals and energy resources South Australia - Volcanoes of the Mount Gambier area - link

- Split shots in Engelbrecht's cave - by Liz Rogers Photography - link