Beehive Casemate is a curiosity rather than a dive site in the usual sense: a piece of Sydney's colonial fortifications that can only be seen from underwater. In the 1870s and 1880s, when fear of Russian raiders drove the building of the Middle Head, Georges Head and South Head batteries, plans were made to lay a controlled minefield across the harbour entrance. The mines would be fired electrically from a casemate – the 'Beehive' – dug into Middle Head, and the firing cables had to reach the water.

The engineers cut a straight channel through the intertidal reef on the headland's southern face, lined it with concrete, and drove a short tunnel from the channel up into the cliff, where a stone staircase climbs to a door and the casemate above. Whether the mines were ever laid is unclear; the infrastructure certainly was. Underwater, the cutting announces itself by walls 'too perfect' to be natural, though kelp on the approach hides it well.

The tunnel runs about 5 m before bending some 30 degrees to the right and continues less than 5 m more to a point where a diver can surface in the dark at the foot of the staircase. The middle section around the bend is properly dark; the ends are lit from the water and from above. At the top a door and possibly a further passage can be seen, but climbing the high steps in scuba gear with surge working in the channel has defeated most visitors.

Depth throughout is only a few metres. Tony Steinbeck's photographs from the 1980s show that divers have known the site for decades, notwithstanding a 2017 media report of its 'discovery'. It carries no marine-life attraction beyond the ordinary Middle Head reef outside – kelp, wrasses, leatherjackets, the odd octopus – and it should be treated as an overhead environment with surge, entered only on a flat day by divers comfortable in confined spaces.

It suits history-minded divers as a ten-minute detour from a Middle Head or Chowder Bay dive; it is within Sydney Harbour National Park and must not be damaged.

On the southern (Chowder Bay) face of Middle Head, a short distance east of Obelisk Beach. Most easily reached by small boat from Chowder Bay/Clifton Gardens (5 minutes) or the Spit/Balmoral (10 minutes); anchor several dozen metres off the cliff and swim in. A determined shore diver could swim east from Obelisk Beach (see Middle Head shore dive) but the cutting is hard to find from the water.

The entrance looks like a concrete-lined drain in the rock at the waterline, often hidden by kelp. Exit the way you came.

The cutting is at the waterline on a rock face open to the south, so any southerly swell or southerly wind chop pushes surge through the channel and tunnel and makes it unsafe; choose a flat day with a northerly or westerly. Mid to high tide is needed to float through the tunnel and reach the staircase without excessive step height, but at very low tide the steps are too high to climb in gear. Visibility follows the harbour – clean on a winter flood, brown after rain.

Boat traffic to Chowder Bay and Obelisk passes close.

Locate the perfectly straight walls of the reef cutting (look for the concrete lining under the kelp), follow it in at 2–3 m to the tunnel mouth, swim the first 5 m to the 30-degree right-hand bend, then the last few metres to surface in the chamber at the foot of the stone staircase. Look up at the stairs and door; do not attempt to climb in gear in surge. Reverse out.

Total time under 20 minutes, usually done as an add-on to a Middle Head or Chowder Bay dive.

Short overhead; torch essential; no line normally needed but surge can push divers against the walls.

No operator. Visited by a handful of private divers and historians; documented by Michael McFadyen in 2017 after a media 'discovery' report, and known to divers since the 1980s.