Green Rock, Koh Tao: Exploring Passages and Pinnacles

Green Rock, located off the northwest coast of Koh Nang Yuan, presents itself as a complex underwater landscape built from large granite boulders and rock formations. The primary feature of the site is its series of natural passages, arches, and small caves, inviting divers to navigate through these tight spaces. This topography creates a challenging yet rewarding dive profile, where attention to buoyancy and trim is rewarded with unique perspectives.

The dive typically begins with a descent onto the main rock formation, which starts around 5 meters below the surface. From there, the site slopes down to a sandy bottom at approximately 32 meters at its deepest point. Traversing these depths means moving between sunlit shallows and the dimmer, more enclosed environments of the swim-throughs. The north-western side extends further with a sequence of scattered rocks leading to the deeper sections.

A significant draw to Green Rock is its resident marine life, particularly the barracudas and moray eels. Schools of yellow-tail barracudas (Sphyraena flavicauda) are often encountered aggregating in numbers, while various species of moray eels (Muraenidae) can be seen tucked into the crevices and under ledges. These larger residents move through the site's structure, offering dynamic encounters as you navigate the corridors.

The site also hosts an array of smaller organisms that thrive within the sheltered areas of the rocks and passages. Look for cleaner shrimps (Stenopus hispidus) and Durban dancing shrimps (Rhynchocinetes durbanensis) within the caves, often found at cleaning stations. An impressive variety of nudibranchs (Nudibranchia) also inhabit the rocks, showcasing vibrant colours and intricate patterns for those willing to look closely.

One cannot dive Green Rock without acknowledging the presence of triggerfish (Balistidae), particularly the Goliath or Titan triggerfish (Balistoides viridescens). These territorial fish are a known characteristic of the site. While generally calm, they can become defensive, especially during nesting season (typically April-May), making it prudent to maintain a respectful distance if you observe signs of aggression near a sandy patch.

The geological composition of Green Rock, with its numerous granite blocks, fosters a diverse invertebrate ecosystem. Giant soft coral fans root themselves on the rock faces, and brightly colored sponges (Porifera) add splashes of colour to the deeper sections. This combination of structural complexity and varied fauna makes each dive a distinct experience, as currents and ambient light shift throughout the day.

Moving through the site’s topography, you'll encounter sections with vibrant corals, particularly along the shallower edges closer to Koh Nang Yuan. Beyond the main rock system, the “Trigger Pit” on the northern side is a notable sandy area. Here, numerous triggerfish establish their nests, highlighting the importance of situational awareness, particularly during breeding periods when they are more protective.