What are the toughest adventures on the planet? Which extreme exploits push willing participants to their mental and physical limits? Depends what you mean by tough, travel and adventure. It's an open question with no wrong answers, but here are a few feats of endurance, endeavour and, dare we say lunacy, fit to stir the most jaded adrenalin junkie.
Rafting the Franklin River
Tasmania is an earthly paradise for lovers of adventure, and rafting the fearsome Franklin River is one of the Apple Isle's ultimate thrills.
The trip down the Franklin, starting at Collingwood River and ending at Sir John Falls, takes between eight and 14 days depending on the state of the river. A 100km journey that begins in the beautiful Irenabyss Gorge ends with the fury of the 5km-long Great Ravine, a stretch of white water which boils with dangerous rapids with giveaway names such as the Cauldron, Thunderush and the Churn.
Experienced rafters can tackle the Franklin independently if they're fully equipped and prepared, but for everyone else (and that's 90% of all those people brave enough to attempt it), there are tour companies offering complete wilderness rafting packages, including World Expeditions, Rafting Tasmania, and Water By Nature.
Run the Marathon des Sables
Considered by some to be the world's toughest foot race, the infamous Marathon des Sables (Marathon of the Sand) consists of no less than six marathons run over six days - in the middle of the Sahara Desert. Sound bonkers? It is. But this masterpiece of masochism still draws devotees from all around the world.
The marathon takes places in Morocco. Competitors must lug all their personal belongings and food in a backpack, while the organisers lay on water and shelter at the end of each stage. Competitors run, walk, or simply stumble a grand total of 250 kilometres, with the longest single stage registering at a palpitation-inducing 88 kilometres in its own right. And they do this in temperatures approaching and even in excess of 50C.
Suffice to say, if you didn't enjoy that sponsored fun run around the local park, this probably won't be your idea of a good time.
Completing the Snowman trek
Widely considered to be one of the hardest treks in the world. Only a handful of people each year attempt this 25-day route through Lunana, the most remote region of Bhutan. It's expensive, gruelling and success is far from certain. Of the few who do set out along the high mountain passes, less than half complete the route because of altitude sickness or heavy snowfall.
The trek, which starts in Drukgyel Dzong and ends in Sephu, crosses 11 passes of more than 4,500m, following trails through yak herder settlements and isolated farms against an eye-bulgingly beautiful backdrop of 7000m Himalayan peaks.
To cap it all off, the window of opportunity for this high altitude undertaking is vanishingly small - the short season when the paths are likely (but not guaranteed) to be open runs from late September to mid-October.
Cycling the Tour d'Afrique
You'll need four months if you want to complete the whole of this peddle-powered trans-African odyssey. First held in 2003, the Tour d'Afrique starts at the Pyramids of Giza and threads its way down through the continent to Table Mountain.
Riders clock up a total of around 11,869 kilometres at an average of more than 112 kilometres a day as they follow the back roads through Egypt, Sudan, Ethiopia, Kenya, Tanzania, Malawi, Zambia, Botswana, Namibia and finally South Africa.
Dehydration is the main danger, but blisters are all but guaranteed, as is a major case of saddle sore. And, of course, the local kids may pelt you with rocks.
On the plus side, the route takes in a handful of Africa's most iconic sights, including the Karnak Temple, the Ngorogoro Crater, the Great Rift Valley, Victoria Falls, Fish River Canyon, the Okavango Delta, and the Dune Sea of the Namib Desert.
Surfing the 'Devil Wave' at Shipstern Bluff
Take a true end-of-the-world location, add huge, unpredictable and just downright dangerous waves, and throw in some hungry great white sharks for good measure - that's the forbidding recipe for Shipstern's Bluff, one of world's most extreme surfing spots.
Once known as the Devil's Point, this remote area off the tip of the Tasman Peninsula, Tasmania, is a challenge to reach let alone surf. Shipstern is only accessible by boat, jet ski or a two-hour walk through the bush. Lugging all your gear, of course.
That means you're a long way from civilisation - and specifically, a hospital - if anything goes pear-shaped. And it frequently does at Shipstern, where the Southern Ocean and the Roaring 40s conspire to whip the cold water into a monstrous right-hander.
Unless you were born on a board and grew up to be a big wave rider who mocks their own sense of mortality, just admire the footage instead.
Doing the other Coast to Coast
The Antipodean version of the Coast to Coast might share its name with a famous meander through the countryside in the north of England, but there the similarities end.
This one traverses the rather more rugged terrain of New Zealand's South Island, including the formidable mountains of the Southern Alps. It starts at Kumara Beach on the Tasman Sea and finishes at Sumner Beach on the Pacific, passing through a backdrop instantly recognisable thanks to Lord of the Rings.
The race involves a 140km stint on a bike (which breaks down into three stages of 55km, 15km and 70km), followed by a 36km run (33km of which crosses the Alps) and finally a 67km kayak down the Grade II whitewater of the Waimakariri Gorge, New Zealand's answer to the Grand Canyon.
Incredibly, the fastest competitors complete this trial in a little under 11 hours. Take heart and stiffen the sinews, though: the oldest competitor to undertake the challenge was 75, while the youngest was just 15.
BASE jumping the Cave of the Swallows
The idea of BASE jumping off anything is enough to send most of us scuttling for the safety of a spot behind the sofa, but what about doing it into the mouth of a cave?
Sótano de las Golondrinas, or the Cave of the Swallows, is a huge limestone sinkhole near Aquismón, San Luis Potosi, Mexico. Its name comes from the many white-collared swifts and green parakeets which live in the holes in its walls. But the spectacle of them exiting and entering every day is not the only reason the cave draws so many visitors.
The Cave of the Swallows is deep enough to accommodate a skyscraper and that has caught the imagination of those who enjoy plunging off fixed objects with only a packed parachute strapped to their back.
It takes about 10 seconds to freefall from the highest point of the cave's mouth to the floor (which, incidentally, is a popular hang out for bugs and snakes).
Don't forget to pull that ripcord amid all the excitement.