The ground beneath a two-storey building in Sydney's south opened last Friday, creating an enormous hole.
This sinkhole, which appeared along West Botany Street in Rockdale, occurred above construction work of the M6 tunnel, with emergency workers being called about 5am.
Speaking at a press conference on Monday, NSW Premier Chris Minns said this part of the project was closest to the surface, meaning there were "major engineering challenges".
"Obviously, engineers from [Roads and Maritime Services] are on site determining what happened here and making sure that lessons are learned and it doesn't happen again," Premier Minns said.
Around 1,800 cubic metres of concrete has since been poured to fill the subsidence.
This sinkhole is the latest in a series of sporadic occurrences across the country.
The cause of the Rockdale sinkhole has not been confirmed.
Others have occurred entirely unrelated to underground construction.
Last July, a sinkhole emerged in the South Australian city of Mount Gambier.
In 2017, a sinkhole opened in Sydney's Point Piper, near former Prime Minister Malcolm Turnbull's mansion.
But what exactly is a sinkhole? How do they occur? And should we be concerned?
What is a sinkhole?
Sinkholes, also known as dolines, occur when a portion of the surface ground sinks into a cavity below, leaving a hole.
Francois Guillard from the University of Sydney's School of Civil Engineering said sinkholes happened when a hole was created underground.
"There are different mechanisms on how this can happen, but eventually the final result is that if the hole gets big enough, the surface layer … is just going to collapse," he said.
Sinkholes occur across the world, but they're more common in areas where rocks and materials known to dissolve in water, such as limestone, are prevalent in the earth.
Both North and South America are known to be common areas for sinkholes.
Sinkholes can happen both in the ground but also in water, such as The Great Blue Hole off the coast of Central American country Belize.
What are the causes?
According to experts, the most common cause of a sinkhole is water.
Guillermo Narsilio, a professor at The University of Melbourne's Department of Infrastructure Engineering, said the causes could vary, "but almost invariably, water is present in its development".
One cause can simply be water currents below the surface slowly creating chasms, working over a timespan ranging from decades through to thousands of years, that grow larger and larger until the surface layer can no longer support itself.
Another cause can be torrential rains, floods, or burst pipes resulting in excess water in the ground.
Sinkholes can also occur without water.
Geophysicist and UNSW associate professor Stuart Raymond Clark said a sinkhole occurred simply when some materials under the ground were eroded or removed.
He said removing material under the surface caused the surface soil to sink into the ground.
While there were measures put in place prior to tunnelling to prevent sinkholes, he said a sudden sinkhole could occur if tunnelling removed "a lot of material", or if the strength of the above ground was overestimated.
"Either there is not enough support or that when they are removing the material through the tunnelling, they think that it is stronger than it is and the rock is not able to support the weight of the ground above it," he said.
Dr Guillard said in urban areas, there was "at least a 50 per cent chance" there was some human contribution to a sinkhole.
Should I be concerned?
Sinkholes are not particularly common in Australia, compared to other areas of the world.
"I've received calls [about sinkholes] maybe once or twice a year," Professor Narsilio said.
"The dangerous part is that it's actually very difficult to know what they are.
"And so they can develop for years without any external sign of this thing happening underground."
Warning signs of sinkholes include fresh cracks in the walls of buildings or nearby ground.
Dr Clark said dramatic collapses in the earth were very hard to predict.
Professor Narsilio said one consequence of increased rain and flooding could be an increase in sinkholes.
Research from the UNSW in 2022 reported that high-intensity rainfall in the Sydney region increased by 40 per cent over a period of 20 years.
Dr Clark agreed, but said it was not as simple as more water causing more naturally occurring sinkholes.
"As that rainwater gets more acidic and there's more CO2 in it, that can accelerate the process.
"But whether [the collapse] happens today or a week from now or a month from now, or ten thousand years from now, they are the kind of timescales these natural things occur on."
He said while climate change will accelerate the natural processes of sinkholes, that doesn't necessarily mean we're at a greater risk of them.
"In terms of human construction-related [sinkholes], we just have to take into account all these different parameters."
Those parameters include heavy rainfall over a period of years.
"Those kinds of things need to be taken into account in terms of estimating the weight and the strength of rocks," Dr Clark said.
"I think we're not particularly at extra risk, due to climate change, as we would be from fires and so forth."
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