Cathedral Cave is the signature dive of the Tasman Peninsula and is regularly described as the largest sea-cave system in Australia. The entrance is a soaring arch in the cliff at the back of Waterfall Bay, broad and high enough that daylight floods the first chamber and divers can hover in open water while looking up into a dome of rock. From there the system tunnels back into smaller caverns, narrow passages and cross-connections.

The names given by the Eaglehawk Dive Centre crew are part of the experience: the Catacombs, tunnels wide enough for one diver at a time that lead to an inner chamber; the Back Door exit; Revelation Bend, where sunlight slants into the passage; Skull Cave, a dark gravel-floored passage with two eye-hole openings; and the Devil's Tonsils, a tight passage that curves upward and needs careful buoyancy. Maximum depth at the entrance is about 21 m and most of the dive is between 10 and 18 m. What makes the cave remarkable is the life on its walls.

Because the rock is shaded and swept by clean oceanic water, invertebrates that normally live in deep water grow here in the shallows: jewel anemones in pink, orange and yellow, golden zoanthids, sponges (including species endemic to the area), ascidians, bryozoans and hydroids. Marine biologist Karen Gowlett-Holmes, co-owner of the dive centre, has recorded species new to science on these walls. Clouds of bullseyes hang in the darker sections, draughtboard sharks rest on ledges and rock lobsters peer from cracks.

The cave is dived as a guided tour. Open Water divers can do the main route with a guide, but good buoyancy control and comfort with an overhead environment are essential, and the deeper or tighter passages are reserved for experienced divers. It is an outstanding photographic site, with the light shafts at the entrance and Revelation Bend the classic wide-angle images, and the wall invertebrates rewarding macro shooters.

The site has been featured in international magazines and is the reason many divers travel to Eaglehawk Neck. Pair it with a second dive in the canyons or on the Waterfall Bay walls, which share the same colourful invertebrate cover without the ceiling.

Boat access only. Drive from Hobart on the Tasman Highway to Sorell, then the Arthur Highway through Dunalley to Eaglehawk Neck (about 75-80 km, 1 hour 15 minutes). Eaglehawk Dive Centre is on Pirates Bay Drive above the bay; divers kit up at the centre and the catamaran leaves from the Pirates Bay jetty/boat ramp at the northern end of the Neck (Old Jetty Road).

Go Dive Tasmania meets at The Adventure Lodge in Taranna and launches at Eaglehawk Neck or Port Arthur depending on conditions. Waterfall Bay is about 8 minutes south by boat from Pirates Bay, past Tasman Arch and Devils Kitchen; boats pick up a mooring or hover in the lee of the cliffs.

Waterfall Bay is the most sheltered part of the Tasman Peninsula's ocean coast and is the site operators fall back on when the Sisters and Hippolytes are blown out, because the bay faces east-north-east and the high cliffs block westerly and south-westerly winds. It is cancelled by easterly and north-easterly winds and by large south-easterly swell wrapping into the bay, which drives heavy surge into the caves and gutters. For cave and swim-through dives the swell needs to be small (well under about 1.5 m) or the surge becomes dangerous.

Heavy rain turns the waterfall on and can put a fresh, tannin-stained layer on the surface for a day or two. Late summer and autumn give the calmest, clearest conditions; winter mornings with westerlies are often excellent.

Descend outside the main entrance at 15-18 m, check buoyancy and torches, then follow the guide in under the arch into the main chamber. The guided route runs back through the Catacombs (single-file tunnels) to the inner chamber and out through the Back Door, with an optional loop past Revelation Bend and Skull Cave. Stay behind the guide, keep off the floor, and finish along the outside wall in 5-10 m before a safety stop against the cliff.

Guided cavern-style route; cave certification is not required for the standard tour but a primary and backup torch are expected. Penetration beyond the daylight zone is limited to the guided route.

Eaglehawk Dive Centre – guided cave dives from its catamaran most days that weather allows, usually paired with a second Waterfall Bay site; Go Dive Tasmania – runs Waterfall Bay on its Tasman Peninsula boat days; Tasmanian Scuba Diving Club – club trips.