Round a bend on a misty New Zealand beach and you’ll stop dead: massive, perfect stone spheres — some over seven feet across — scattered on the sand like alien eggs or giant marbles dropped by passing titans.

This deep dive explores the Moeraki Boulders on Koekohe Beach, geological marvels millions of years in the making. We weave together the poetic Maori oral tradition, the deep-time science of how they formed molecule by molecule, and the enduring mysteries that even modern instruments can’t fully crack.

• The Maori legend of the canoe Araiteuru, in which the boulders are petrified eel baskets, gourds, and sweet potatoes washed ashore from a shipwreck.
• How these septarian concretions grew through mass diffusion in still marine mud, forming perfect spheres over 4 to 5.5 million years under up to 160 feet of seabed.
• Their anatomy: a hard calcite “candy shell,” a weak interior riddled with cracks, and a hollow core lined with spiky scalenohedral crystals.
• Why the origin of the boulders’ internal septaria cracks remains a genuinely unresolved geological mystery despite X-ray and microprobe analysis.
• Their relatives — the Koutu and Katiki boulders — including Katiki concretions that grew around the fossilized bones of mosasaurs and plesiosaurs.