Middle Arch (Rikoriko Cave) – Poor Knights Islands Marine Reserve

20m Max

10-25m Vis

boat

Site Overview

Middle Arch, also known as Rikoriko Cave, is the world's largest sea cave by volume and one of the most extraordinary dive sites at the Poor Knights Islands Marine Reserve. The cave is so massive that its interior could contain several cathedrals, with the main chamber reaching approximately 100 metres in length and 30 metres in height. The cave entrance is a vast opening that admits natural light into the first section, but as you swim deeper, the cave transitions into complete darkness where torch light reveals an entirely different world of colourful marine life thriving on the walls. The cave walls are covered in lush soft corals, sponges, and anemones that create a vibrant underwater garden in the torch beam. Moray eels occupy crevices throughout the cave system, while stingrays rest on the sandy cave floor in the deeper sections. The acoustic properties of the cave are remarkable, with sounds echoing and amplifying in ways that create an otherworldly atmosphere. Swim-throughs connect different chambers of the cave, each revealing new marine communities and geological formations. The cave's immense size means that multiple dives are needed to fully explore its different sections, and each visit reveals new passages and features. Despite the overhead environment, the cave is spacious enough that divers never feel confined, and the multiple entrances and exits ensure safe diving practices. Rikoriko Cave is a truly unique diving experience that combines geological wonder with marine biodiversity in a way found nowhere else on Earth.

Dive Briefing

"The dive begins at the massive cave entrance at approximately 5 metres, where natural light illuminates the first chamber and demoiselles swim among the colourful wall growth. Switch on your torch as you swim deeper into the cave, where the light gradually diminishes and the torch reveals the full vibrancy of the soft coral and sponge gardens on the walls. Follow the main passage into the largest chamber, where the ceiling rises to 30 metres above and the walls stretch away in all directions. Moray eels are abundant in the wall crevices, often extending their heads to observe passing divers. Stingrays rest on the sandy cave floor in the deeper sections at 12 to 15 metres. Continue through the swim-through passages that connect to secondary chambers, each with different marine communities and geological features. The cave walls host an extraordinary diversity of nudibranchs and small invertebrates that are best appreciated with close torch inspection. The return follows a shallower route through the upper passages at 5 to 8 metres where the light returns gradually. Exit the cave through a secondary opening that provides a different perspective of the cave system. The safety stop is conducted near the cave entrance where the light creates beautiful patterns on the rocky substrate."