Gripping a rubber hose between his teeth that will be his lifeline, Than Hlaing takes a last gulp of relatively untainted air before plunging into the murky gloom of the fast-flowing Yangon river in Myanmar's main city.
Lashed with weights to help him dive straight down, the 40-year-old peers through the heavily silted waters as he descends to the riverbed.
There, he will grope in the darkness several metres below the surface for the treasure that has given him an income for a decade -- coal.
"We think that they spilled from coal ships many years ago," he told AFP after a recent trip on the river, although he is unsure of the exact origin of the coal.
Up to 20 boats bob on the churning brown waters, swollen from the daily torrential rains.
Divers, breathing air tainted with the diesel fumes from the generator that pumps it through the rubber pipe, feel blindly along the riverbed, which Than Hlaing says is as "smooth as concrete".
Once a lump of coal is found, the divers load it into a simple woven basket to be hauled onto the ship. The chunks can be as big as footballs.
"Our main dangers are when the air pipe is blocked and when the boat's engine breaks down," Than Hlaing told AFP.
Coal is a crucial commodity in Myanmar, where around two thirds of the population do not have access to electricity despite the country's rich natural resources.
Local markets sell piles of charcoal and coal stoves are the norm in rural areas and streetside food stalls.
The International Energy Agency estimated that in 2011 some 92 percent of the population relied on traditional cooking techniques using biomass -- which includes fossil fuels such as coal as well as plant material. That compares to 26 percent in neighbouring Thailand.
Myanmar's monsoon season is thought to be the best time for divers to find coal, as the rains bring faster flowing currents and better visibility in the water.
But it also increases the risk to divers.
Than Hlaing estimates they are able to haul up between 160 and 800 kilogrammes (350 and 1,760 pounds) a day during those few months.
It is a family business -- fathers, cousins, brothers all dive together. The crew of about six earn 22 US cents per kilogramme and in peak season can make up to $175 a day.
Even once it is divided up, it can be a good wage in a nation where more than a quarter of the population live below the poverty line of $1.25 a day.
In quieter times they hire themselves out as salvage divers for other ships.
"Sometimes we find bits of iron in the water and after selling them, we can earn a small amount of money for motor oil for the boat. It's not much," said Than Hlaing.
"Some people have found mystery things, old things from ages ago but I've never found one."