On Groote Eylandt, off the NT's Arnhem Land coast, a fine dust coats the houses and cars.
It gets under fingernails and coats people's roofs.
For years, the black and brown dust has blown off the South32 Gemco manganese mine.
Locals who live and work on the island suspect the fine dust particles could be threatening their health, and they want answers.
Sylvia Tkac used to work at the South32 Gemco manganese mine. She also lived in the community of Angurugu.
"When you fly over the island, when you look at your community, all you can see is black dust on the roofs," she said.
"I went to clean my mother's house and the dust is so thick, people are using Gernis to clean their houses."
A few years ago, researchers at the University of Queensland tested Ms Tkac's hair and nails to see if there were traces of manganese in them.
The tests found what the local Aboriginal Land Council called "concerningly high" levels of heavy metals.
"I was quite shocked to see the results," she said.
She now lives in Darwin, but she is worried about her family members who are still living beside the mine.
"[South32] say they do monitor the dust, and they have equipment in the community to see how much is coming in, but we don't get told the results. Where does that information go?"
Ms Tkac is calling on the Northern Territory government to investigate whether the dust is threatening the community's health.
"We are worried about the future of our children, so we'd like them to do more testing and we'd like to know what positive programs they can put in place to stop this dust," she said.
The test results revealed by the ABC in 2021, and community concerns about dust levels, prompted some action by the mining company South32.
The mining company helped pay to seal roads, changed its mining schedules and moved stockpiles, and said it was using watering trucks to reduce dust on its mine site.
But since then, the Health Department has not investigated the issue.
People on Groote Eylandt are calling for health testing by government authorities and more action to prevent dust.
Local man makes concerning discovery
Most of the 3,000 people who live on Groote Eylandt are Indigenous.
The rest include mine workers, many of whom live in the mining port community of Alyangula.
Residents have long been aware of the threats posed by the dust they can see.
But David Nathan, who until last year worked as a linguist on the island, believes he may have discovered evidence of dangerous air pollution from particles called PM2.5, which are too small to see.
These PM2.5 particles are most commonly produced in Australia by traffic pollution, bushfire smoke and industrial emissions.
Sized 2.5 micrometres and smaller, they can penetrate deep into the respiratory and cardiovascular systems.
In comparison, a human hair is about 70 micrometres in diameter.
In 2020, Mr Nathan began monitoring PM2.5 levels over Alyangula on a weather-forecasting and air-quality website called Windy.com.
"Having lived in Japan, I was quite aware of some of the air pollution issues that they face there, and I noticed on Windy.com there were these very strong plumes of PM2.5 emanating from Groote," Mr Nathan said.
"And it is very intense. Some of the readings were well over 100 micrograms per cubic metre — up to eight times safe levels, either for a long period or repeated daily over a long period."
The Australian safety standard limit for PM2.5 is 25 micrograms.
Mr Nathan saw readings on Windy.com over Groote Eylandt's port town of Alyangula of between 94 and 273 micrograms in January, February, March, September and October of 2020.
In December 2022, Windy.com also showed readings of 56 and 75 for PM2.5 over Groote.
He is calling for an investigation into whether some of these high fine-particle readings could have been caused by the loading of particularly fine manganese dust called "fines" at the South32 port loading area at Alyangula.
"We observed these fine materials in the sheds being turned over, perhaps to dry them out," he said.
"That may be the source, these so-called fines being blown off a conveyor belt, but that would need to be investigated.
"I think it would be good if suitably qualified environmental science or medical people could liaise with Windy.com to establish the credibility of the readings on their site which appear to show massive unsafe levels of PM2.5 over Groote."
The data shown on the Windy.com website is sourced from the European Union's Copernicus Atmosphere Monitoring Service.
The director of the Copernicus service, Vincent-Henri Peuch, told the ABC that in remote areas like the NT there was a margin for error, and the service could not tell exactly what the PM2.5 material was.
This is because the service uses both air-monitoring data from ground-monitoring stations reported to governments around the world, and observations from its own satellite system to create its air-quality readings.
Unlike in Australian cities, where air-quality information is collected from ground-monitoring stations, in remote areas like the NT, the service has to rely mostly on observations from its satellites.
But he said industrial and traffic pollution could be differentiated from bushfire smoke by cross-referencing air-quality data with data about where heat spots are caused by fires.
"In the case of the NT, we don't have ground observations," he said.
Mine says dust exceedences have been limited
South32 did not dispute the readings or provide its own monitoring data when the ABC provided the company with them.
However, it said it had only limited PM2.5 and PM10 dust exceedences from its operation before July 2022, and none since.
"We … have worked hard to successfully prevent further occurrences," the company said.
"The health and wellbeing of those living in Angurugu, Alyangula and the broader Groote Eylandt community is extremely important to us."
The company said it held quarterly meetings with Groote Eylandt's Aninindilyakwa Land Council to discuss the results of its air-sampling program.
The NT's Industry Department, which is responsible for regulating the mine, told the ABC no PM2.5 exceedences caused by mining activity had been reported to it between 2021 and 2023.
It also said there could be other reasons for high readings.
"Many factors outside of mining activities attribute to the exceedences of National Environment Protection Measure guidelines, a key influence being bushfires," it said.
The ABC consulted three PM2.5 experts from different Australian universities who all said high readings of PM2.5 near communities, if correct, would be a concern.
Clare Murphy, a professor of atmospheric chemistry from the University of Wollongong, said the levels on the maps were worrying.
She has used satellite services like Copernicus, and she said while they did have a margin for error, their strength was showing where there were problems.
"So you may not be able to necessarily believe the numbers exactly but you can see where the pollution is bad and where it's high, and the absolute correct response to this is to get something on the ground to understand what's happening so it can be rectified."
The NT Environment Protection Authority told the ABC it was not investigating any dust issues on Groote Eylandt.
