He's a sulphur miner, one of a rare band of men who harvest the element from within the crater of Ijen.

The Ijen volcano complex dominates the landscape of easternmost Java. There are several volcanic cones, rising to 2,799 metres, and active craters include that of Ijen itself, which, for decades, has been mined for its sulphur, a bright yellow element that is common in volcanic gases.

During the Industrial Revolution (late 18th to early 19th centuries) demand for sulphur, which was used in gunpowder, soared. It was mined extensively in Sicily, southern Italy, first by digging from open ground, then by tunnelling in hot, hazardous conditions. Sulphur mining has occurred elsewhere, such as on a volcanic island in New Zealand and high in the South American Andes. Yet nowadays sulphur is a by-product of oil refining and other industries, and only in Indonesia is it still mined by hand.

At one Javan volcano, miners dig sulphur from the slopes, then trundle it downhill on roughly built wooden wheelbarrows. Here at Ijen, there were attempts to use horses for transportation but the terrain proved too tough for them, so miners not only extract sulphur, but also carry it downhill in bamboo baskets.

A narrow dirt road of compacted black stones and soil heads towards the crater. It winds steeply upwards and to the left and right bushes and tall trees are picked out by torch beams. Faint yet foul aromas of hydrogen sulphide, the gas of rotten eggs, waft in from time to time.

The trail reaches a wooden hut - a weighing station, where miners check how much they are carrying. A miner is resting here and offers for sale little figures made of sulphur.

The path narrows, climbing a steeper slope. There are steep drop-offs to the darkness on the right. The vegetation is lower, with trees more stunted. Then, there are no trees at all, just skeletal remains of shrubs, all that is left after an outpouring of toxic gas. The sulphurous smell is stronger now and the path makes for the rim of the crater.

The crater lies below, a gaping black hole. There are stars above and the lights of a distant town are visible way down and beyond the volcano. But looking into the crater and hoping to see the blue flames that are said to dance within, I'm disappointed to find nothing but blackness.

A rough trail leads to what seems like a rocky spur jutting out towards the crater. It ends at a vantage looking almost vertically down. And there, shining brightly below, are the blue flames of Ijen. Deep in the crater, close together, are three or four patches of fire, with what seems like smoke around and above them, sometimes swirling to obscure the eerie flames. There are other patches of light moving, from the torches of miners and visitors roaming near the fires.

"Do you want to go down?" asks a miner who has been waiting by the path. We make our way towards the fires, some 200 metres below.

The path twists and turns downwards, across small crags and plunging slopes of loose stones. Some years ago, a tourist fell to his death here, so I tread carefully. The stench of rotten eggs becomes more intense and there's another acrid odour - sulphur dioxide, which is likewise toxic. I pull on the paper face mask my guide has given me.

It's still only 4am, but already some visitors are making the return trip up the trail. "It's a tough climb - but worth it," an Australian man says.

I stop to give miners space as they slowly ascend, each heaving two baskets filled with chunks of sulphur and slung from a pole supported on a shoulder.

Some miners are resting, their baskets placed on rocks that must serve as regular stopping places. It can take each well over an hour to reach the crater rim, yet even this pace is astonishingly fast given their loads may weigh 70kg. Imagine carrying that kind of weight any distance in a hi-tech backpack over level ground, never mind in two baskets balanced on a pole up a rock face.

The slope eases at the bottom of the crater wall and we walk to a stony ridge, a natural viewing gallery from which to watch miners at work just tens of metres away.

It's still pitch black, helping the weirdly wonderful blue flames stand out. Patches of flame illuminate what looks like a living hillside. Steam is blasting out all around the flames, then surging upwards to form a great column that disappears into the night sky. There's a continuous roaring sound, like that of a giant furnace.

The "fire and brimstone" horror described in the Bible may have originated with volcanoes, and their brims, as brimstone is an old name for sulphur.

Eight miners are at work in front of us, moving below the sulphurous steam, using head-mounted torches to locate fresh deposits, which they smash and prise from the rock surface with metal poles. One wears a gas mask but others just use pieces of cloth over their mouths, or no protection at all.

I have read that the life expectancy for sulphur miners is just 30. This figure seems too low to me, as I see miners who are clearly older, but few will reach what most of us would consider a ripe old age.

Dawning daylight illuminates rows of ceramic pipes running down the steaming slope. They have been installed to capture gases emerging from fumaroles (openings in the Earth's crust) and guide them downwards, so they cool a little, allowing sulphur to condense and trickle to collection points. Gases from fumaroles can reach 600 degrees Celsius.

We move further down and are soon at a working area beneath the surging steam. The ground underfoot is yellow with sulphur powder. Men are moving between collection points, where intensely red liquid sulphur drips from pipes and turns yellow as it solidifies. Some are breaking it with the rods, gathering chunks; others pick up fragments.

My guide - who usually works down here, ensuring production remains solid - fills small moulds with liquid sulphur, to make the souvenirs miners offer visitors: a crab, a turtle, a cat.

Thanks to a remarkably concentrated mix of sulphuric and hydrochloric acids, the water in the sullen green crater lake below me is the strength of battery acid. Almost 200 metres deep, a kilometre wide and with a volume of 3 million cubic metres - equivalent to over a thousand Olympic-size swimming pools - it is the world's largest acidic lake. In 1817, its basin collapsed and mudflows destroyed three villages. In 1976, a sudden release of gas from the lake killed 11 miners.

About 400 men work at the mine, my guide says.

"Most carry 70kg of sulphur, twice a day," he says.

For this, they earn about HK$80 - double the amount labourers make in nearby coffee plantations.

"For working on production, I earn a small salary, but it's not enough to support my family, so I usually also carry sulphur with me as I leave."

My guide is short, perhaps as a result of heavy loads having effected his spine. He pulls his shirt collar down, revealing rough red patches on a shoulder worn by the pole between the baskets. Yet he is smiling as we part; life does not have him beaten.

Two nearby volcanic cones, forested hillsides and a plain below make for superb scenery as we walk down the outside of Ijen. The miners, oblivious to the views, are sure-footed on the steep slope, taking short quick steps, most shod in basic sandals. Often, they're moving at almost jogging pace and at intervals dip a little beneath the baskets, using swift hand movements to switch the shoulder the pole is resting on. Each halts at the weighing station, to check their load, and most smoke a cigarette before heading down again.

At the bottom of the path - three kilometres from the rim - is the official weighing station, run by a nearby sugar refinery that oversees the mining operation. Here, each miner watches as his load is weighed and money owed is calculated. The sulphur is then piled into a waiting truck, which will take it to a simple factory, where it will be melted and cooled in a refining process. Ijen sulphur may then be used to make sulphur dioxide for whitening sugar.

The 30 or so miners slumped around the weighing station look exhausted, but many will be returning to the crater, aiming to make a second trip before the midday heat makes conditions too unbearable even for these astonishingly tough men.