Cologne Gardens is one of the most-dived sites on Cocos and one of the most versatile: the official tourism listing calls it a popular sea wall suitable for all levels, with a range of 9–30 m. The reef top at 9–12 m is covered with 'blooming' hard and soft corals – a dense garden that keeps shallow divers and snorkellers occupied – while the wall drops away below it to deeper water where orange gorgonian sea fans grow. Underwater Australasia's Cocos feature describes gorgonians at 35 m and below, and Diveplanit reports fans nearly the height of a diver, so the biggest fans sit at the deep end of the range, with rose wall corals between them.

Batfish colonies hang off the wall, and the site is known for two seasonal spectacles. Around the full moon, large schools of blacktail snapper gather here, and in summer bumphead parrotfish arrive to breed – big, heavy-set fish moving in groups across the reef top, crunching coral as they go. Between those events it is simply a healthy, colourful tropical wall with the usual Cocos cast of grey and whitetip reef sharks, trevally and a wide variety of reef fish.

Its position on the western side of the atoll puts it in the lee of the south-east trades, which is why it is dived so often through the windy months. The layout means a group of mixed experience can dive together: newer divers stay on the reef top and upper wall, while experienced divers drop down to the fans. The wall is not sheer everywhere – parts are a steep slope – and the reef top is broad enough to spend a whole dive on.

For photographers, the orange gorgonians against blue water are the prize and are best lit with strobes; the reef-top corals suit natural-light wide-angle on sunny days. The name, like many on Cocos, comes from local diving lore rather than any chart.

Fly to West Island on the twice-weekly Virgin Australia service from Perth (usually routed via Christmas Island; allow 7–10 days on the islands to absorb weather and flight delays). Cocos Dive collects divers from West Island accommodation in the morning (pickup around 8 am, departure about 9 am) and runs its boats from West Island into the lagoon. The north-western sites around Horsburgh Island are among the closest to West Island: the boat runs north up the inside of the lagoon past the northern tip of West Island towards Horsburgh, a short, usually sheltered ride.

Moorings or live-boat drops are used; there is no shore access. The lunchtime surface interval is normally taken on the beach at Direction Island before the second dive.

Cocos has two seasons. The south-east trades blow steadily from roughly April/May to October, often 15–25 knots, which chops up the lagoon and pushes swell onto the eastern and southern outer reefs; divers are then kept to sheltered sites. The doldrum season (November to March/April) brings lighter, variable winds, flatter seas and the widest choice of outer-reef sites, and is the best time to dive, though it is also the tropical cyclone season and squalls pass through.

The north-western rim sits in the lee of the atoll during south-east trade winds, so these sites are often the ones that stay diveable when the trades blow; a strong westerly or north-westerly swell (uncommon, mostly in the doldrum season or with passing cyclones) closes them. Tidal flow through the Western Boat Passage near Horsburgh sets up currents along the reef edge, so dives are timed with the tide; visibility is best on the incoming tide when clear oceanic water pushes in.

Begin on the reef top at 9–12 m among the coral bommies, then move over the lip and descend the wall to the gorgonian fans at 25–30 m. Spend the deep part of the dive among the fans, then ascend slowly along the wall, finishing across the reef top and completing the safety stop over the coral.

Cocos Dive (Dieter Gerhard, West Island, operating since 1990) is the only scuba operator on the islands and the only way to dive this site. It runs seasonal two-dive day trips with lunch and a surface interval on Direction Island, maximum 10 divers per trip; sites are chosen on the day according to wind, tide and diver experience. Trips book out well ahead and courses are currently suspended, so arrive certified.