Thunder Point is the south-eastern corner of Lady Julia Percy Island, the point that takes the brunt of the Southern Ocean swell, and the Blowhole is the sea cave that gives it its name. In ordinary weather every swell that hits the point drives water into a cave in the cliff and blows it out through the roof with a boom that can be heard across the island; on the rare day when the sea is glassy, the Scuba Doctor records, there is underwater an 18 m wide opening to a magnificent cave penetrating deep under the island. Few have ever seen it and fewer have dived it, and the site guide calls it a truly awesome experience. The entrance lies on a basalt wall dropping into 25 m, and the wall itself, in cold upwelled water, is heavily encrusted with sponges and ascidians and patrolled by the island's seals and its schools of sweep, butterfly perch and boarfish. Leopard Seal Reef, with its giant underwater rock pools, lies a few hundred metres to the north-east along the same southern cliffs.

This is an expedition dive in every sense. It requires a flat calm on the most exposed side of an island 8 km offshore, a boat skipper willing to hold position under the cliff, and divers with cavern experience, lines and lights who understand that a swell arriving while they are inside would be lethal. Nobody should plan a trip around it; it is something to be attempted when a party already at the island for the walls and seals finds the point silent and the sea like glass. For the diver who gets that day, it is one of the most extraordinary places on the Victorian coast, a cave under a seal island that almost nobody has entered. The island is a Wildlife Reserve and Indigenous Protected Area, landing is prohibited without a permit, and great white sharks frequent the colony.

Boat only, from Port Fairy (Griffiths Street ramp, about 20 km, 40–60 minutes) or the Yambuk river ramp, to the south-eastern end of the island. Thunder Point is the exposed corner and the cave is obvious in any swell because it blows; when it is not blowing and the sea is glassy, the boat holds off the cliff and divers drop directly over the entrance. Leopard Seal Reef lies 384 m to the north-east along the southern cliffs.

Landing is prohibited.

Lady Julia Percy Island stands 8 km offshore in the open Southern Ocean and the crossing from Port Fairy or Yambuk is exposed in any weather; the island offers a lee on one side or the other but nowhere is calm in a swell over about 1.5 m. Plan for a south-west swell under 1–1.5 m and winds under 15 knots, choosing sites on the northern (Landing Bay) side in southerly weather and the southern side in northerlies. The Bonney Upwelling brings cold, nutrient-rich water from November to April, when visibility can drop and water temperatures fall to 13–14 °C even in summer; autumn generally gives the clearest water.

Great white sharks frequent the colony, particularly during the pupping season from November to January. Diveable perhaps 30–40 days a year for a boat based in Port Fairy. The Blowhole is diveable only when conditions are, in the Scuba Doctor's words, really flat and calm with no swell – any swell at all surges through the cave and drives water out of the blowhole above.

That is a handful of days a year.

Drop over the cave mouth, descend the wall to the entrance at about 15–20 m and enter only with a line, lights and cavern training, staying within sight of the entrance light. Explore the entrance chamber and the wall either side, turn on a third of your gas, exit the way you came and ascend the wall to a stop in the lee for a pick-up. In any surge, dive the wall outside and leave the cave alone.

Cavern/cave penetration – line, primary and backup lights, rule of thirds, cavern or cave training essential; do not enter in any surge. Depth to 25 m at the entrance suits air or EAN32.

No regular charter. Port Fairy and Portland fishing charters take divers to the island by arrangement in settled weather; Warrnambool Diving and Melbourne clubs organise occasional trips; most dives are from private boats out of Port Fairy or the Yambuk (Eumeralla River) ramp. The Scuba Doctor notes that few have ever seen the cave and fewer have dived it.