The Tunnel is one of a trio of cave dives at the southern end of Black Rock, alongside The Arch and The Chimney. McFadyen describes a reef that drops from 9 m to a sandy bottom at 21 m, with a wall of rock coming down to the sand and a distinctive cutting in it. The Tunnel itself is quite long – perhaps 15–20 m – and does not end blind; it exits into a vertical hole that rises to the top of the reef at about 12 m. The Marine Life Network describes a 15 m cave with a collapsed roof, about 4 m high and 3 m wide, with sea fans and plenty of fish, at 22–24 m.

The rock here is igneous and covered in a striking purple coralline growth, with red, orange and yellow sponges, sea squirts and gorgonians. McFadyen describes eastern blue devilfish so tame they would take food from the hand – feeding is now discouraged – and wobbegongs and Port Jackson sharks often rest in the tunnel.

The dive has a satisfying shape: along a wall, through a tunnel and up a chimney-like exit onto the reef top, then back across the reef. The tunnel is roomy enough for most divers, with light at both ends, but it is still an overhead environment and suits Advanced divers with good buoyancy and a torch. Combined with The Chimney nearby, it makes a good two-dive day at the south end of Black Rock, only minutes from the ramp.

Boat access only. Black Rock lies just off Lilli Pilli on the southern shore of Batemans Bay; the nearest ramp is at Mosquito Bay (Lilli Pilli), a couple of minutes' run, and Beneath the Bay brings divers from the Hanging Rock ramp in Batemans Bay in about 15 minutes. McFadyen's mark is immediately south of Black Rock; boats anchor on the ridge to the east of the main part of the site, which keeps the anchor off the tunnel roof.

The southern end of Black Rock is a little more exposed than the western side but still gets shelter from southerly swell. Easterly and north-easterly swell surges through the tunnel and vertical hole and makes the dive unpleasant; avoid it in swell over about 1.5 m from the east. Visibility falls after heavy rain.

Best in westerly or southerly weather.

Descend from the ridge to the wall and follow it west and then north to a cutting in the reef. Swim through the cutting into The Tunnel at about 18 m, follow it 15–20 m and ascend through the vertical hole to the reef top at 12 m. Swim east across the reef and return along the southern wall to the anchor.

Beneath the Bay dives The Tunnel and The Chimney at Black Rock's southern end (reef drop to 21 m with soft corals). Spirit Divers also listed it. Visiting clubs dive it on Batemans Bay weekends.