Resting in the mountains north of Tokyo is the small town of Kusatsu, home to one of Japan’s most beautiful hot springs or “onsen.” Here, the thermal water flows naturally from the active volcano, Mount Shirane, and is passed through Japanese cypress-wood channels to bring the temperature down to a (still toasty) 48 degrees – tolerable thanks to the high-mineral content. The key to the water’s healing powers lies in its high acidity – an aluminium yen coin would dissolve in the pools in less than a week. Although this might not have you rushing to dip your toes in, the waters are renowned for their antibacterial healing properties – rumour has it the only ailment they can’t cure is heartache.

The Maldives are pretty flawless – pale sands, azure-blue waters, towering palm trees etc, etc, etc. Now forget all that. At Huvafen Fushi they’ve bypassed the beach and set their sights on the seabed. One of the world’s only underwater spas, Lime is an all-glass space that tends toward with avant-garde (and has a treatment menu to match). Banish jet lag with a bubbling marine-mud body wrap, detox with a spirulina scrub after a cocktail-fuelled evening or prep your skin for a day on the beach with a homemade lime and sea-salt exfoliation. Ariel ain’t got nothing on you, honey.

Clinical spa experience this is not. One of New York City’s most under-the-radar spas, this Tribeca-based subterranean retreat is as atmospheric as you’ll find this side of the Hudson (although they have also sprouted other locations in Seville, Barcelona and Almeria). Located in a former textile factory built in 1883, the Aire Ancient Baths is most suited to those who like a good soak. Fusing traditions from the Greek, Roman, Ottoman and Arab empires, the prevailing idea here is the Romans’ custom of hopping from frigidaria to caldaria (i.e. progressing from cold to hot pools). Slippers are provided, caverns are flooded with candles (the only source of light) and all you’ll need to pack is your swimsuit.

A programme advertised as having “no options” is indicative of it’s hardcore approach to wellbeing. Mandy Moore, Lea Michele and Selma Blair have all sweated it out at this week-long luxury boot camp in the mountains of Malibu. Words of caution: if you are a crier, came last in the bleep test at school or are a bit of a Malibu Barbie, you might want to sit this one out. At The Ranch it is all about personal awareness and life transformation – physically, mentally and spiritually. You won’t eat a lot – not unless you consider six almonds a proper snack. Still, vegans and plant-based dieters will coo over The Ranch’s signature granola with almond milk (its scrumptious – or is that the starvation talking?). Food rationing aside, The Ranch is a results-driven destination. Trainers wake guests at 5.30am each day for their morning four-hour hike. And that’s just a warm up.

Set in Mondrian London at Sea Containers, Agua makes for a particularly glamorous spa experience (guests of the hotel are given complimentary access, as well as to the gym). Go with a gang and set the relaxation room as your group meeting point. Centring on a singular golden droplet, the surrounds sets the tone for the decadent afternoon in store. If your nails are in need of a little TLC post massage and steam, The Glamour Room is a must. Preen, prettify and pamper before heading to the Rumpus Rooms on the rooftop for an Aperol spritz – by which time you’ll be too relaxed to worry about your trite drinks order.

Get your body going with free guided walks and daily yoga sessions at COMO Shambhala Estate. With an emphasis on holistic wellness, a week at this retreat likely to include lots of pilates, Ashtanga and mountain biking along mountain trails on Mount Batu. A 15-minute drive away from Ubud, COMO brings things back to nature (with a luxurious slant). Cosset in your thatched one-bedroom Retreat Villa – each with a private treatment room, ideal for those planning to make regular spa trips. COMO’s Ojas spa fuses the best of Eastern and Western therapies; move seamlessly from Swedish massage to Ayurveda over a day of back-to-back appointments. In between activities, grab a pressed juice or organic salad from all-day restaurant, Glow. Come dinner, guests can design their own three-course menu to cook with the chef. Bespoke bliss.

Don’t come here if your idea of zenning out involves all-white surroundings. Tierra Santa is awash with rainbow-hued rugs, a pom-pom chandelier, bright Juan Gatti artwork and floral-printed pouffes. Located on the third floor of Faena Hotel, the spa boasts one of the largest hammams on the East Coast. Indulge in a full wet-spa experience – think herbal steam room, sauna and ice parlour – and book in for the Hamam Rose Ritual; a unique treatment which involves lying on top of a warm marble slab while a therapist scrubs you down. Post rigorous exfoliation you’ll be covered in soft foam, then a soothing clay and conclude with a massage. Inspired by South American and South Asian healing traditions, Tierra Santa’s treatments are far reaching. Led by two partnering doctors, Tierra Santa also has a preventive medicine program, including testing, exams, detox and digestive health offerings. Weekly spa treatments are far more justifiable when they’re listed under “doctor’s orders”.

Buchinger Wilhelmi needs no introduction. A European health destination famed for its intrinsic belief in the power of fasting (*gargles with epsom salts*). Best to get on board with the idea of elimination – a daily enema is par for the course here. Dedicated to cleansing, the Buchinger clinic is committed to sorting out your gut, cleansing your mind and addressing your emotions. For grub, you’ll dine on surprisingly delicious light soups for your initial fasting days before returning to solids. Above all, Buchinger Wilhelmi is a medical facility, so blood tests, thyroid checks and personal training are likely to be on the agenda. After a week of daily guided walks, therapy sessions and delicate eating you’ll leave feeling lighter (in every way).