From the surface, you might never know much of Newcastle in New South Wales has been built on top of mines — some more than 100 years old.
But for residents across the Hunter region, gaping holes in the backyard, depressions in roads and paths, and other subsidence issues are a consistent reminder.
"You always hear about walls that have sunk in a corner of a home, or a foundation had collapsed, or something's on the move," Wallsend Heritage Group president Kavil Martin says.
Ms Martin grew up in Wallsend, a town dominated by mining in the 19th century.
"Wallsend, Fletcher, Maryland, Minmi, all those areas are totally undermined," she says.
"You're lucky if you don't get subsidence in your backyard, really."
When the collapse of an underground mine shaft led to a major subsidence event on Fogo Street in Wallsend last week, up to 40 people were forced from their homes.
More than a dozen were unable to return for several days, but have all since returned.
Subsidence Advisory NSW says it is conducting geotechnical investigation work along the street to learn about the condition of the mine and what remediation work is needed.
Two months earlier in February, a large sinkhole appeared about 1-kilometre away on Kenrick Street.
Subsidence Advisory of NSW spokesperson Valerie Griswold says while sinkholes occur all over the country where abandoned mines have been covered over, it's a significant issue in the Hunter.
"Sinkholes are so common in the Newcastle area that they have a mascot, Maurie the Mole," she says.
Mine collapses
Coal mining began in Newcastle in the early 1800s, and during the 19th century, the region became the most important coal port in the southern hemisphere.
Demand for coal transformed the region with dozens of underground mines established and a web of railway lines built to carry coal to the harbour.
Newcastle and many surrounding suburbs now sit above the old mines.
While some have been filled in, there are many parts that haven't been and over time they collapse.
Development across much of Newcastle must be approved by Subsidence Advisory NSW.
In some instances, work to fill underground mines must be carried out to ensure structures are safe.
Remediation work
Engineering geophysicist Jonathan Fitzgerald runs a company that conducts subsidence studies and remediates sinkholes.
He says he'd like to see more proactive steps taken to avoid subsidence occurring.
"Unfortunately a lot of the work we do is reactive," he says.
"In Canberra, they were considering fixing radars to rubbish trucks and things like that, so as they're travelling the streets, they are obtaining data.
"The radars can see the dipping layers within the soil of the formation beginning and then we can start to take preventative remedial action."
However, he acknowledges not all collapses can be predicted.
"There are lots of warning signs that happen prior to collapse, but sometimes they can be very instantaneous as well," he says.
"And what we're seeing now is the issue of all the flooding ... that's basically opening up a lot of these sorts of areas."
Geotechnical engineer Stephen Fityus says considering the mass amounts of coal extracted from the region over the years, it wouldn't be feasible to fill all the "holes" left behind by mines.
"If you think of all the holes that must be down there, it's not really practical on the environmental coast," he says.
"Finding stuff on the surface to put back down the hole and then doing that would be absolutely enormous."
