They seem immovable - mighty and unchanging.
Spat out of the Earth's crust, the balls of grey stone - 60 million years in the making - known as the Moeraki Boulders sit impassively on the edge of the Otago coast, occupying a small stretch of sand and surf just north of Moeraki.
They are not magnetic, yet they draw tourists from around New Zealand and the world to the same patch of coastline each year.
The collection of more than 50 boulders, and the small tourist industry that has developed around them, do not appear to be going anywhere.
Weighing up to seven tonnes, and measuring up to 2.2m across, they stand as rocky sentinels watching over the sea.
Maori waka and European whalers have come and gone, fishing vessels from Moeraki pass by and Pacific Ocean storms drive in, but the boulders remain.
Yet, for all their impassiveness, their days are numbered - relatively soon, at least geologically speaking, the Moeraki Boulders will disappear for good.
That was the message from GNS Science and University of Otago experts in Dunedin when asked to predict the boulders' likely lifespan.
GNS Science geologist Jane Forsyth said the boulders visible on the beach - called "septarian concretions" by geologists - were only the visible portion of many more still buried further inland, as well as others believed to be concealed in the seabed offshore.
The constant pounding of the Pacific Ocean was contributing to the slow disintegration of the boulders on the beach, but also revealing more of the treasures over time, she said.
"As the coastal cliff keeps getting eroded and the material slumps forward, as far as we know, they [boulders] are just going to keep on emerging - being born like eggs out of a chicken."
However, eventually, the supply of fresh eggs was going to cease, she said.
The rock formation that had produced the boulders extended "many kilometres" north, south and inland from the boulders' location, she said.
The process began with the gradual accumulation of underwater muddy sediment over 60 million years, which over time gathered within it a collection of shells, small bits of wood and other organic material, she said.
Eventually, minerals within the sediment began to harden into rock, gathering around the fragments of shell, wood and other organic materials, which acted as nuclei, and cemented together by calcite crystals, she said.
Without the organic material as a focal point, a sheet of rock would result.
With the organic material in the mix, spheres of rock began to grow.
Their smoothness was dependent on the stability of the growing environment over many millennia.
A small boulder could take 120,000 years to grow, while the largest at Moeraki could take four million years, she said.
Time-consuming? "Not to a geologist," she said.
After all the growing, something else began to happen - the boulders hardened, dehydrated from the inside, and began to crack.
The cracks filled with calcite, resulting in yellow veins on the boulders, she said.
And, once exposed to nature's battering on the beach, those cracks were exposed as weak points - the cracks expanded and the boulders began to dissolve, first into chunks of rock, then fragments, and eventually into sand, she said.
The first signs of deterioration in the boulders, once on the beach, could be evident in as little as five years, she said.
However, an average-sized boulder's cycle from beginning to end - starting when the formed boulder rolled from the cliff on to the beach - could take hundreds, or even thousands, of years, she said.
"It's logical to think in the hundreds of years timeframe they can be born, lie on the sand and be "worn down into sand.
"It's of that order - hundreds, maybe thousands, but not millions [of years]," she said.
The boulders served as a focal point for tourists, many of whom seemed to be on a pilgrimage to clamber aboard and pose for photographs.
However, all the foot traffic was unlikely to be speeding the boulders' demise, she believed.
"I would think twice daily inundation by the tide, and probably weekly storm surges . . . is likely to be far more relevant to how they break up than anybody with a pair of running shoes standing on top of them for a photo."
Boulders taken in the years before the area became a reserve, and their uplifting illegal, could be found decorating the entrance to homes in nearby Moeraki, and even at the university campus in Dunedin, she said.
However, they were typically in better condition than those left to the mercy of the coastal environment, she believed.
Associate Prof Daphne Lee, of the University of Otago Department of Geology, said early paintings and photographs of the area would show there were "lots and lots" of boulders at one time - perhaps more than 100.
Many had disappeared in less than two centuries, either taken by souvenir-hunters or smashed apart by the waves, she said.
About 50 visible boulders remained.
She had been taking first-year geology students on field trips to the boulders for 25 years and, in that time, had witnessed the birth of new boulders, including one that slowly emerged from the cliff over a period of five years.
It was a process that could also predispose the boulders to eventual decay, if a bumpy roll down the cliff - and possible collision with boulders already at the bottom - encouraged cracking, she said.
However, the Moeraki Boulders were not unique as some thought.
Concretions, to use the scientific term, were "quite common" - including at Shag Point, south of Moeraki, she said.
It was inside one of the area's concretions the fossil remains of a 7m plesiosaur, together with a smaller mosasaur, were found.
What was unique about the Moeraki examples was their near-perfect symmetry, helped by a particularly benign growing environment at the time they formed, she said.
"They must have just grown in all directions . . . in very quiet times," she said.
However, the process that produced them was no longer occurring at Moeraki as a result of sea-level change, meaning the number of boulders - on the beach or still buried inland - was finite, and destined to run out one day, she said.
The cliff edge was remorselessly moving inland - 1000 years ago, it would have been 100m or more further out to sea, she said.
In another 1000 years, erosion would have pushed the coastline further inland, exposing more the boulders and closing in on the last one.
"Going forward in time another 1000 years, I would guess they would probably still be there.
"In 20,000 years, I'm not sure."
Eventually, they would be replaced, elsewhere.
The process that produced them might have stopped at Moeraki, but it was continuing in other locations, including outside Otago Harbour, where smaller and misshapen concretions - some the size of a human fist - had already been found, she said.
And, in the meantime, Ms Forsyth had a simple message for those travelling to view Moeraki's rocky treasures.
"I would say appreciate them - just appreciate them, because they are fantastic," she said.
The boulders
Formation began some 60 million years ago.
More than 50 boulders visible in the area.
Largest weighs 7 tonnes, measures 2.2m across.
Two-thirds measure over 1m.
Smallest measures about 30cm.
Many more boulders buried in the area.
Boulders now a major tourist attraction.
Source: GNS Science.
