New Zealand has been under the sea for millions of years during the past eons. It is up and down based on the pressure being applied to the edges of the continental plates of which it is part. Actually New Zealand is at a junction of two plates and it slides, bumps, jumps, and changes its elevation over time. Mountains are being built on New Zealand’s west coast (South Island) as you read this page and those same mountains are being eroded away at an equal speed. There are dozens, if not hundreds, of earthquakes every day. It is a geologically exciting place to live but perhaps not a great place to display a collection of fine China.
The undersea portion of New Zealand’s existence has allowed for the formation of a great deal of lime-based rock. The ocean oozes shells and calcium rich bits that form layer upon layer on the ocean floor. The layers reach a certain weight and undergo a chemical and physical change. This divides the build-up of ooze into what look like separately formed layers of calcium carbonate (the mineral calcite). Some day, millions of years in the future, those “layers” are/will be lifted above sea level. Being a rather wet part of the world these limestone edifices are rained on a great deal. The fresh water seeps down into the myriad of cracks and through the layers of calcite eroding the limestone and carrying off an aqueous solution rich in calcium. Picture that happening for tens of thousands of years and you have a subterranean cave. New Zealand has miles and miles of such caves. The drips will sometimes evaporate before dripping – this leaves behind the calcium which can create a structure on the walls or ceiling of the cave.