On a stretch of coast between Oamaru and Dunedin on South Island lays a collection of around 50 big round boulders, the Moeraki Boulders. They come in different sizes, the largest more than 2 meters in diameter. The big ones weigh up to 7 tons. They look a bit like old style footballs, patches of dark leather stitched together with a lighter coloured thread, just look at the exploded ones.

They were formed about 60 million years ago, and the large ones took some 4 million years to form. Calcium salts assembled around a solid hard centre, but remarkably, the very heart of the stones now seems empty, as can be seen with the ones that burst open.

There are similar boulders, the Koutu Boulders, around Hokiango Harbour on NZ’s North Island.