The fall could be five metres or 50 metres; there’s no way of seeing over the ledge below. Not that it matters – any debilitating injury could prove fatal, because being carried out of Mount Tambora’s crater is not an option.

I bear that in mind as a 15-metre traverse with footholds threatens to collapse, tufts of grass the only handholds available. Knees buckling, thighs aching, baking hot, long out of water, overburdened with a heavy pack and seriously questioning myself for having accepted this challenge, there is nowhere to go but down.

A friend and I had been planning this trip for months when Mount Agung, on the Indonesian island of Bali, 275km to the west, began threatening to explode, rain down destruction and close the surrounding airspace. But that could have been hours, days, months or even years away, so we took the gamble in the hope that, if successful, we would join the relatively small group of people – perhaps fewer than 50, excluding porters – who have descended into the crater produced by the most powerful volcanic eruption in modern history.

Tambora, the highest mountain on the Indonesian island of Sumbawa, was responsible, in 1815, for the only confirmed seven on the eight-point Volcanic Explosivity Index since AD180. Its crater is 6km in diameter and drops to a depth of 1,100 metres. Pre-eruption, the mountain stood at 4,300 metres; after the cataclysmic event, it measures just 2,851 metres.

After remains of a house and its occupants were dug from beneath the solidified ash in 2004, Tambora earned comparisons with Italy’s Mount Vesuvius, which buried Pompeii in AD79. The massive eruption of Tambora caused some 11,000 deaths initially, and an estimated 49,000 worldwide through ensuing famine and epidemic diseases. Gases, ash and dust reached the stratosphere, where they remained long enough to filter the sun and earn 1816 the sobriquet “the year without summer”.

Mary Shelley, who was on holiday in Switzerland at the time, described it as “a wet, ungenial summer”, in the introduction to Frankenstein. The “incessant rain often confined us for days to the house”, and with nothing better to do, the novelist put pen to paper and produced one of the great works of English-language fiction.

English artist J. M. W. Turner apparently found inspiration in Tambora’s effects, too: it was at this time that he began depicting in his paintings unusually spectacular sunsets, which would have been produced by fragmental material released into the atmosphere by the eruption.

Crops failed and livestock died in much of the northern hemisphere in the months following the explosion, trigger­ing the worst famine of the 19th century, and the unseason­ably wet and cold weather may have played a part in the defeat of Napoleon at Waterloo, in June 1815, about two months after the climax of the eruption.

It was with some trepidation, then, that I accepted an invitation from Peter Day, a former Hong Kong resident who now lives in Bali, to trek to Tambora’s summit and descend into its caldera.

As the two of us set off on the five-day journey, accom­panied by five local guides/porters, Dutchman Rik Stoetman, who, with his wife, Nural, runs the Visit Tambora trekking company from a small village perched on the slopes of the volcano, promises that the descent and ascent of the crater wall will be pretty easy for a reasonably fit person. What he doesn’t mention is that a climb to the summit of Tambora is as far as most of his clients are prepared to go.

Managing to avoid the poison-ivy-like plants that, with the slightest contact, cause days of itchiness and throbbing, the first section is a delightful seven-hour hike, giving the adventurer time to admire the changing scenery, from lower jungle and meandering streams to old-growth forest.

Clear water filters out of the bank of a stream through a hastily stuffed length of PVC driven into the soil from the last source of water we encounter before we go over the lip of the caldera. From here, it’s a three-hour hike, one night’s camp and a seven-hour descent until the next refill, we’re told.

After a 3am start the next day, thinning vegetation gradually gives way to lava paths and ruts, slippery rocks and loose earth, as we strike out for the summit.

Standing on the peak as the sun comes up, trying to imagine the scene in 1815 when earth, rock and lava were blown sky high, is awe-inspiring. As the sun illuminates more and more of the caldera, details become clearer: gas rising from the edges of the crater, a murky lake some 100 metres in diameter, trees, shrubs and, on the far side, Anak Tambora, or “Tambora’s child”, a six-metre-high gaseous lump, 30 metres in diameter and with the consistency of mud, that is slowly pushing out volcanic rock, proving that this stratovolcano remains alive and well.

After three hours of slipping, sliding and the occa­sional tumble under a fierce sun, we lunch on a rockfall.

Out come the nylon ropes, five in total, which are as thick as a finger and knotted to provide some grip. They are tied to the roots of trees no broader than a wrist and we descend hand over hand.

Many of the boulders here are as big as a minibus, and under one, which is balanced neatly across others, is a cave of sorts in which we hope to find fresh water. After some minutes of digging in the sand, though, we realise the rains have not produced enough run-off for water to pool; not good news when we have just one can of soft drink remaining between us.

Three hours later, exhausted, with legs that no longer listen and water uppermost in our minds, we set foot on the caldera floor. “Martian” is the best description of the landscape. Deep (but dry) riverbeds scar the ground, boulders are strewn as far as the eye can see and rock outcrops have been dyed white and yellow by continuously rising sulphur-laden gases.

The walk to the centre of the crater, where we set up camp, is another long hike. There, we make the most wonderful discovery: a foot-wide stream that has gouged its way through the sharp volcanic sand, running over a perfectly white rock bed. Water never tasted so good.

Vegetation grows all the way down to the lake, in the corner of the crater, and apparently animals thrive here, too. On the first of our two mornings in the caldera, a porter returns to camp with a deer’s leg, dry as a bone, which he sets about slicing up with a machete: instant jerky.

The caldera floor is hot and dusty, and a constant wind flattens the tents, the only shelter from the relentless sun. As well as forcing volcanic sand into the smallest crevices, the particle-laden wind, caused by hot air rising and more air being sucked in underneath, makes for average photo­graphs. All this way for half-baked images!

That night, the mid-autumn full moon, cast against a cloudless sky – combined with a lack of wind, and therefore dust – comes to the rescue. The landscape is transformed: the stream becomes liquid silver; the clarity and stillness of the air allow the barren landscape to be appreciated in peace.

The bowl of starry sky cradled by the crater’s walls; the booms as rocks and debris slide down the rock faces; the caldera illuminated by lunar light; the Mars-like landscape ... all is quite surreal.

Then, the moon ducks behind the volcano walls, leaving us in utter darkness.