Popes Eye is an artificial island that was never finished. In the 1880s the colony of Victoria began a ring fort on a shoal 4 km inside the Heads as part of the defence of Port Phillip; a horseshoe of bluestone blocks was laid – the Scuba Doctor listing gives its dimensions as about 204 m long, 22.4 m wide at the base tapering to 1.3 m at the top and rising 2.8 m above low water – before advances in gunnery made it redundant and work stopped. Left alone, it became a reef, and since 1979 it has been a marine reserve; it is now part of Port Phillip Heads Marine National Park and the oldest protected piece of seabed in Victoria.

The horseshoe faces roughly north-east, and the dive has two parts. Inside the annulus the water is 1.5–4 m deep, calm and full of fish that have never been hunted: old wives, scalyfin, zebrafish, sweep, leatherjackets, magpie perch and big bluethroat wrasse crowd around divers and snorkellers. Outside, the bluestone drops to sand at about 10 m (14 m at the deepest) and the blocks are covered in gorgonian fans, feather stars, sponges and kelp, with sea urchins, abalone and nudibranchs among them. A ship's anchor lies at 12 m on the ebb side and an old wooden butter churn on the flood side, both useful for navigation. Giant cuttlefish are regular in winter, and Australian fur seals from Chinamans Hat 2.5 km to the east visit constantly; above the water a colony of Australasian gannets nests on the platform, the only mainland-attached gannet colony in Australia.

Popes Eye is the beginner's Heads dive and the peninsula's snorkel site, but experienced divers should not skip it: the fish density and approachability are unlike anywhere else in the bay, and the structure is a reliable night dive. It is dived on almost any tide, though the current outside the walls is strong on springs and the start of the ebb is best.

Boat access only, 15 minutes from either Portsea Pier or Queenscliff Harbour and one of the few Heads sites reachable by small private boat in most weather (Sorrento or Queenscliff ramps, 15–20 minutes). The annulus is a Parks Victoria-managed structure with a gannet colony on top: landing is prohibited, and boats moor or drop divers outside the walls, usually on the western (ebb) side. The West Channel shipping lane passes close to the east, so a dive flag is mandatory and skippers watch for the ferry and channel traffic.

Snorkel and dolphin-swim boats visit throughout the day in summer.

It can be dived on most tides, but slack water gives the easiest conditions and the best visibility. Slack water at Port Phillip Heads does not coincide with high or low water: it falls roughly three hours after high or low water at Point Lonsdale, and the usable window is typically 20–40 minutes. Use the Ports Victoria / Port of Melbourne slack-water predictions for The Heads (also republished by charter operators and WillyWeather 'Rip Bank') rather than a standard tide table.

Inside the Heads it is sheltered from most swell, but a south-west swell over about 2 m still surges across the top of the wall and a westerly or south-westerly wind over 15–20 knots makes the boat work unpleasant; big seas outside drop visibility to a few metres for a day or two. Visibility is best in late summer and autumn (Feb–May) when the water is clear and the westerlies have eased; winter brings 11–13 °C water and bigger swells but often the clearest days after a run of settled weather. Heavy rain has little effect this close to the ocean.

Scuba Doctor lists it for slack water, ebb and flood – it is the one Heads site that can be dived on a running tide because the structure gives shelter, but the current outside the walls runs hard on springs and the best conditions are the start of the ebb. Diveable in almost any wind; a strong southerly makes the boat ride wet.

Descend on the outside of the wall in 8–10 m on the sheltered side, circle the annulus clockwise or anticlockwise with the bluestone on one shoulder – look for the ship's anchor at 12 m on the ebb side and the old butter churn on the flood side as reference points – then finish inside the horseshoe in 2–4 m among the fish before ascending in the lee. Total circuit about 250 m.

Dive Victoria Group (Portsea/Queenscliff) – daily as a training, second or snorkel site; Bayplay (Portsea) – daily in season, dive and snorkel; Red Boats (Queenscliff) – frequent; Sea All Dolphin Swims (Queenscliff) and Moonraker (Sorrento) – snorkel tours; Polperro Dolphin Swims – snorkel.