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10 million years ago the volcano that created these islands was 25 km in diameter and stood 1000 metres above the surrounding landscape.
An eruption formed a giant gas bubble that became Rikoriko Cave
How do we know it is the biggest in the world?
Using a sophisticated 3D laser scanner for the above water section of the cave, and traditional hydrographic surveying techniques for the seafloor, above and below water models were calculated as well as a volume for combined surface.
The survey of Rikoriko in itself was significant in setting a standard for accuracy and mathematical analysis that might be applied elsewhere in the world.
Its calculated volume is 221, 494 cubic metres (or 7.8 million cubic feet).
This is over twice the volume of two other known sea caves that claim biggest status.
The sea surface inside is roughly a hectare.
The cave is 130 metres long, a depth below water of 26 metres, and a height to the ceiling above water of 35 metres, and a width of 80 metres.
Rikoriko means waning light in Maori, twilight, or dancing light due to the patterns that the sunlight makes when reflected off the surface of the water upon the cave ceiling.
There is plant life in the cave that is different from anywhere else. There are ferns hanging from the ceiling that only receive light for photosynthesis from sunlight coming in the entrance and bouncing off the water surface up to the roof.
At the rear of the cave there is an underwater cup coral that normally grows at depths of 200 metres. The amount of light that reaches the coral, tricks it into thinking it is deeper than it is, so it grows at about 10-15 metres.
The under water life out at the Poor Knights is very lush and the kelp is deeply forested. Inside the cave however, there is no kelp growing at all. The seascape is very stark and moon like.
The visibility is intense and lucid. Visibility underwater is measured in the distance that you can see horizontally through the water. Normal conditions at the Poor Knights can give us between 15 and 25 metres, but inside Rikoriko we often get 35-45 metres visibility.