The Oparara Basin is one of the most famous granite valleys in New Zealand and contains the largest natural limestone arch in the southern hemisphere: The Oparara Arch. Measuring 220 m in length, this limestone formation clearly outdoes any of its sister arches such as the Moria Gate Arch which name was inspired through the Lord of the Rings trilogy.
30 to 40 million years ago, the Oparara River started shaping the limestone deposit by carving out great arches and deep cave networks. The Honeycomb Hill caves, for example, consist of a 14 km long cave system containing a great number of stalactites and stalagmites. Some of those caves are not even fully explored yet but are considered to be great sources of ancient fossils and bones.
The basin and its caves offer home to the endangered giant carnivorous snails, the Powelliphanta species, several local spider species such as the Oparara karamea, Oparara vallus and the largest cave spider of them all: the Spelunga cavernicola that measures a leg span of 13 to 15 cm. The rare lesser short-tailed bats (Mystacina tuberculata tuberculata), blue ducks, giant cave weta and the great spotted kiwi (Apteryx haastii) have also been recorded in the surrounding rainforest.