Iddlebiddy Cave is an Advanced Cave site in the ForestrySA estate that the CDAA describes, somewhat disarmingly, as a very easy dive for competent cave divers. It consists of a single large passage running south-east for roughly 250 m from the entrance chamber and ending in a small, silty room within a clay deposit. Only a couple of minor areas prevent divers swimming side by side.

The passage is the attraction: long and wide, with large areas of flat ceiling and huge boulders, it gives a sense of scale that many Mount Gambier caves lack. The fragility is the reason for its grade. The cave contains delicate features, especially areas of easily damaged tessellated clay and very soft walls, and the CDAA rules are explicit – do not disturb the entrance debris cone, do not fin or touch the delicate clay blocks, and make all gear adjustments away from the entrance.

Iddlebiddy was first explored in late 1983 and early 1984 and has been open to Advanced Cave members since late 1990. In 1988 human skeletal fragments estimated at around 100 years old were recovered from the entrance debris cone, a reminder that many Mount Gambier holes have histories on land as well as underwater. Depth figures are not published on the listing. It suits Advanced Cave divers who want a long, straightforward and beautiful passage and who can move through it without leaving a trace. Many divers rate it among the most beautiful passages in the region. Photographers value the flat ceilings and boulders, but lighting set-ups must be handled well away from the clay.

Book online through the ForestrySA caving portal (forestrysa.com.au/caving – select the cave and date, then enter diver details); ForestrySA recreation staff can help by email (recreation@forestrysa.com.au) or on 08 8391 8800 during office hours. Collect the key as instructed (Mount Gambier Visitor Information Centre for most forestry sites), then drive into the Green Triangle pine plantations on forestry tracks using the booking directions. Park at the site, keep tracks clear for fire vehicles, and carry gear to the entrance.

Forestry sites are closed on declared Total Fire Ban days.

Underground sites are unaffected by swell or wind; the constraints are the ForestrySA booking, the key, and fire danger – all forestry sites close on Total Fire Ban days (frequent December-March) and fires are prohibited 1 November-30 April. Heavy rain turns forest tracks slippery and can wash debris into entrances. Water is a constant ~15°C, so long cave dives need drysuits.

Snakes are active near entrances in warm weather.

Enter without disturbing the debris cone, move away from the entrance before any gear adjustments, then follow the permanent line along the single wide passage south-east for up to about 250 m to the small silty end room. Keep off the tessellated clay and soft walls; turn on thirds.

CDAA grade: Advanced Cave (5L250), owner ForestrySA. Do not disturb the entrance debris cone; no gear adjustments near entrance; no contact with clay. Continuous guideline, thirds, three lights; frog kick essential.

No operator – CDAA Advanced Cave members via ForestrySA booking.