The East Extension, known as EEX, is one of the most significant cave discoveries in Mount Gambier in decades. In 2019 divers pushing beyond the main chamber rockpile of Engelbrecht East found a major new network of submerged passage under the city, and the CDAA now lists it as a separate Advanced Cave site. It offers more than 400 m of submerged tunnel with a maximum depth of about 22 m – considerably deeper and longer than the shallow East passage that leads to it.

It is a sidemount-only site, and the named features explain why. The Rabbit Hole is a tight restriction that the CDAA warns poses a drowning hazard if attempted without preparation and correct gear configuration, and steel cylinders larger than 7 L are banned there. The Caterpillar tunnel is so silt-sensitive that even minimal disturbance reduces visibility to zero. Seasonal water-level fluctuations change the clearances. Cave-rated divers who dive the East passage must stop at the rockpile – they are not permitted into the Extension.

Because of those risks, EEX is managed tightly: a maximum of four divers per timeslot, two timeslots a day, a minimum of two per booking, sign-in with site staff, and a ban on all training. For experienced Advanced Cave sidemount divers it is a rare chance to dive relatively new passage in a city-centre cave; for everyone else it is a reminder that the easy East passage is only the doorway. Because EEX is reached through the East passage, every dive is effectively two dives in one: a shallow silty approach, then a committed sidemount push, so plan gas and time around the full distance from the lake.

Engelbrecht Cave is on Chute Street in central Mount Gambier, a few minutes from the Jubilee Highway. Collect one of the two site keys from the Lady Nelson Visitor Centre, 35 Jubilee Highway East, during business hours, then park at the Chute Street site. Sign in at the site and tell the café/tour staff you are diving, unlock the gate and lock it behind you.

Carry gear down the steps and accessway (built for the dry tours) through the doline to the water; viewing platforms sit at the lakes in both passages. Diving is permitted 8 am-8 pm daily; return the key afterwards (after-hours return is possible). The East Extension has its own timeslots (8 am-2 pm and 2 pm-8 pm, max four divers).

Weather has no effect underground, but heavy rain matters: storm water drains into the doline and can contaminate the first sump, turning visibility from clear to zero, and water levels fluctuate seasonally, which changes airspace and restrictions. Avoid diving for several days after big winter storms. The site is a public tourist attraction with dry cave tours, so weekday mornings are quieter.

Water is a constant ~15°C. Seasonal water-level changes affect the Rabbit Hole restriction in particular.

Pass through the Engelbrecht East passage to the main chamber rockpile, then continue on the line into the Extension. Negotiate the Rabbit Hole with cylinders under 7 L (steel), move through the Caterpillar tunnel without touching anything, and explore the 400 m-plus tunnel to a maximum 22 m. Turn on strict thirds; expect zero visibility on exit if anyone has silted the Caterpillar.

CDAA grade: Advanced Cave (5L19), sidemount only; steel cylinders max 7 L for the Rabbit Hole; max 4 divers per slot (08:00-14:00, 14:00-20:00); min 2 per booking; no training. Continuous guideline, thirds or stricter, three lights, helmet-mounted lights recommended.

No operator; training of any kind is prohibited. CDAA Advanced Cave sidemount divers only, max four per timeslot.