A torrent of water drops 90 feet down a near-vertical rock face, at roughly one bathtub's worth per second. The walls of the canyon rise steeply on either side. A child perches on the brink of the waterfall, wearing a harness with a single rope attached. Given the circumstances, the rope has an interesting characteristic: it is only 60ft long.
The child is lowered gently down two-thirds of the way, their body protected from the slippery limestone by a wetsuit and life jacket. Then the rope is released, and gravity takes charge.
This is the climax of a day's canyoning, itself the alarming culmination of a week of family hyperactivity in the wild Soca valley. The river carves an emerald course through the north-western corner of Slovenia, where the Balkans collide with the Italians (who know the waterway as the Isonzo). Whatever you call the river, it provides a sinuous spine for six days of adventure on land and in water, in spectacular surroundings rich in history and raw in nature. The family comfort zone is quickly left behind – starting early in the week, when you and your offspring are turned upside-down in kayaks.
"You need to know how to escape if you capsize," insisted Klemen, one of the guides responsibe for stretching individual limits while minimising danger. He had taken us to a stretch of the Soca where the river broadens out. It forms a turquoise lake that reflects mighty mountains draped in pine. But appreciating the scenery is tricky when you are underwater beneath a kayak and trying to escape by the approved method.
Kayaks are rendered watertight by means of an elasticated "spray skirt" that clings to your waist and is fastened around the cavity. It's an asset while you proceed in a shipshape and upright fashion but when a rock or eddy tips you up, it becomes an obstacle between you and safety.
"The best way is to feel for the loop at the front of the skirt," said Klemen. "Next best, push sharply up with the knees to release the cover."
When it was my turn to be inverted, I was relieved to discover that the "panic method" also works after a fashion: with enough flailing of arms and legs you can pull yourself free and reach the surface. But Klemen made me capsize again to get it right. I duly held my breath for a couple of seconds while locating the kayak equivalent of the ejector-seat lever. Spills before thrills.
What happened next in our brief training course was inspired. The eight of us – two families – played "kayak ball", a bit like water polo only involving hurling a football around from canoes, trying to score goals by hoiking it into rowing boat. The activity stopped us obsessing about remaining upright in a kayak and started us having fun. It worked: on the afternoon session on the wild water, most remained upright, while those who did turn over put their training to good use. By the end of the day we were dragging kayaks on to a five-foot rock in mid river, climbing in and getting shoved over the edge.
My wife, Charlotte, and our daughters Daisy (11) and Poppy (nine), shared the week with a family from Lincolnshire: Doug and Annette Unkles and their children Joe (15) and Emily (12). We all stayed in a pension in a pretty village outside the non-pretty town of Tolmin. Each morning at nine, a yellow minibus turned up to take us to adventure HQ to be kitted out for the day's challenge. More often than not this involved a wetsuit, life jacket, yellow helmet and rubber boots. But not, fortunately, on mountain-biking day.
A snake of cyclists meandered on paths beside the water and through meadows scattered with wildflowers. To complete the picture of rural bliss, a fawn scampered across ahead of the peloton. Halfway through the day, we donned swimming costumes at the riverside where the locals congregate to dive into the pristine – and chilly – waters of the Soca. We took it in turns to leap from a board jutting from a rock, while the guides leapt perhaps 50 feet from a neighbouring bridge to put our efforts in perspective.
Bridge-jumping apart, the week was an exercise in trust, with the guides calibrating our capacity for thrills and, inevitably, spills. Early in the week, we boarded a pair of rafts, and were instructed to kneel on the inflated tubes and paddle furiously in a race with the other vessel. A carefully spun story persuaded us that this was the best position to adopt. In fact, it was a ruse. Klemen and Peter ran the length of the raft, tipping us into the water in quick succession, to see who reacted with amusement and who with alarm. This was psychological triage, to see who needed to be watched when the going got rough.
Extreme experiences demand extreme catering, which is where Renata comes in. The kindly host of the Penzion Kobala, she serves delicious local produce for breakfast and dinner in the industrial quantities necessary to sustain a week in the frontline of adventure. She also sells beer. Doug and I soon realised that every sip of Lasko we took was saving us money, compared with UK prices. The local merlot and cabernet sauvignon helped to sustain a sense of well-being.
Wednesday was officially a day off, though with adrenalin still pumping, sitting around the pool at the pension did not appeal. The Unkles family evidently had an insatiable appetite for extreme activities: they went paragliding. A convenient 3,500ft mountain behind the hotel provides a launch pad for the €110 tandem take-off, swooping through the thermals before landing in a field next to the local hypermarket.
I chose to explore the region's tragic history. This crumpled corner of Europe looks as though the gods have sharpened their claws upon it. Narrow paths thread through forests of beech and pine, which abruptly end in vertiginous gradients – often scarred by crumbling military defences. The Soca valley began the 20th century as part of the Austro-Hungarian empire. But when the Italians joined the First World War in 1915, it became the arena for some of the most intense fighting of the conflict, in which half a million men died. For two years, the men deployed here stared death in the face every day. This was the Somme, with mountains.
Despite the repeated failure of Italy's attacks, in the post-war carve-up it was awarded the valley of death. The town of Kobarid, a short way upstream, is dominated by an ossuary for the fallen, built on Mussolini's orders in 1938. The museum details the horrific conditions the soldiers endured, and the dreadful death toll. One officer's brisk daily report noted that four of his men had died "falling from the rock face". War cemeteries, some decorated with Hungarian flags placed by families of the fallen, abound.
The Second World War made the Soca valley a frontline once again. It emerged in 1945 as part of Yugoslavia, a messy experiment in federation on the unlucky side of the Iron Curtain. Slabs of awful architecture still blight many towns.
Since Slovenia successfully seceded in 1991, slipping out of Yugoslavia before the bloodshed began in earnest, it has blossomed into a gentle go-between separating Italy and Austria from the Balkans. But by the time you have scrambled across a few wobbly rope bridges across gaping chasms, "gentle" may not be the word you use.
Not everything in the early 20th century involved destruction: the most heroic railway project of the Austro-Hungarian empire cut a line from the imperial heartland to the sea at Trieste, and still serves Slovenia well. The road to the final day's canyoning winds beneath a sweeping rail viaduct that still constitutes the prime link through the north-west of the nation – and provides a temporary diversion from focusing on the challenge ahead.
At the end of the road, we squeezed and grumbled into our wetsuits and made a half-hour uphill hike, on a path that occasionally diminished into a mere gouge in a cliff, as August heated up.
Then, the fun began. On a succession of increasingly challenging cascades, we were taught to slide safely down a rockface – feet first, usually, though with the option of the head-first "Superman" position for the fearless. Peter and Klemen rapidly coaxed us through the rapids. By this stage in the week, we had faith in their judgement – and their strength, should things go awry. So, each of us willingly slithered around a curved rock into the unknown, and found ourselves 15ft above a pool, into which we accelerated at 32ft per second per second. Imagine a particularly elaborate theme-park flume ride, then subtract the machinery and safety measures.
All too soon we were at the brink of that 90ft waterfall. The boys had the measure of the Calder clan: be brisk, be bossy, and provide a helpful shove to overcome anxiety. In quick succession, Daisy and Poppy shrieked when they hit the water – happily, with sheer exhilaration.
My turn. Vertigo deferred to Peter's calm authority: "Keep your legs apart while we lower you. At the point we stop, you have three seconds to put your legs together and relax before we let go of the rope." Slip, slide, splash: which is how, a week ago, I learned to let go and love gravity.
The closest airports are Trieste, served by Ryanair from Birmingham and Stansted, and Ljubljana, served by easyJet from Stansted and by Adria Airways from Luton and Manchester. Simon Calder paid £160 per person return on easyJet.
He paid a total of £2,997 for a family of four, half-board, with transfers from Trieste or Ljubljana, and all activities (except paragliding, for which €110pp is charged locally) through Green World Holidays (01926 330 223; greenworldholidays. com). There is availability for departures on each of the next Saturdays, 18 and 25 August, at the same price.
The local operator is Maya (00 386 5 380 0530; maya.si).
Slovenian Tourist Board, London (0870 225 53 05; slovenia.info).