Is it a cannonball, a medieval mace, a counterweight - or even a meteorite that crashed to Earth?
We invited nzherald.co.nz readers to speculate on the origins of the strange metal ball found by an earth worker at the Grey River shingle pit at Kaiata.
Westroads worker Chris Chamberlain was screening gravel when he noticed the rusted ball among the gravel.
"It looks like a miniature mine but I think it is far too small for a mine," Mr Chamberlain said.
"At first I thought it was an odd looking rock. I'm quite fascinated with odd bits of metal which come across the screen and when I had a closer look I saw it was rusted.
"It's not big and definitely not a mine, but I hope History House can work out what it actually is and what it was used for."
The majority of nzherald readers who chimed in on the debate thought it was a cannonball, with one reader suggesting it was French in origin, while another suggested it was used in the land wars between British colonialists and Maori.
Others thought it was a mace - but most provided no explanation as to why a medieval weapon might turn up in a West Coast shingle pit.
One of the more inventive suggestions was that the ball was used to play bocce volo - an Italian bowling game in which players toss metal balls, similar to petanque. The reader cited the characteristic dimples protruding from the ball as evidence.
Other suggestions included that it was a meteorite, a chimney cleaning weight, a land marker, the ball from a prisoner's ball-and-chain, or a water-pump handle counterweight.
But perhaps the most likely suggestion, put forward by several readers, was that the rusted ball was used for quarrying or mining - a theory that seems to accord with where the ball was located.
One reader said it was likely a grinding ball used in a ball mill, which are used to grind up rocks into powder, often at gold mines.
Another reader said: "Looks like grinding media from a mine. You put these inside a mill to help crush rock. We have similar sized ones at the mine where I work but without the dimples."
Similar grinding balls were used in the limestone industry, another reader said: "Remember seeing drums of them at the lime crushing works in Oamaru when I was younger."
Mystery solved?