A first-timer's guide to Japan's onsens: Where, when and how to do them

The ridge lines of Mount Tsurumi peak out from behind a thick haze and bathe in the amber sunrise. I'm in Beppu on the 15th floor of the glamorous Amanek Yula-Re Hotel in an infinity rooftop pool like no other. This one is full of glimmering, soft simple spring water, which has flowed down from the imposing range along with another 130,000 tonnes in the past 24 hours. Claimed to be the world's second-largest and Japan's most extensive catchment area of hot spring water, Oita prefecture has been regarded as a mecca of onsen bathing for centuries. In the distance, plumes of white steam spew from a scattering of chimneys across the grey cityscape. Appearing motionless, they merely hint at the unrelenting activity underground.

I'm new to the world of onsen and - eager to immerse myself, I head to a well-known steam bath in one of the sub-regions within Beppu Hatto (hot spring areas) called Kannawa Mushiyu. Legend has it this once-volatile part of Beppu was uninhabitable until Buddhist monk Ippen Shonin pacified the torrents with prayer in the late 13th century. There is a statue of him nearby where visitors are invited to pray and, with a wooden spoon, pour spring water over the corresponding body part that ails them. I douse the figure completely.

"Take off your clothes, wash, put on this and wait," says a petite, cheerful old lady as she passes me a gown. Stripped bare, I wander into the onsen pool room, where a row of stools and shower heads sit beside a small steaming pool. I perch on my undersized chair and wash myself in anticipation. In my gown, or yukata, I'm led to a short wooden door and asked to enter and lie down. Inside, it's as if I've entered an oversized stone pizza oven with straw-like bedding on the ground. Crouched low, I lie on the dry mattress in semi-darkness and close my eyes. I read beforehand this is sekisho, a hard-to-source grass laden with medicinal properties known to promote longevity, and purposely inhale the herbaceous aroma with gusto. My mind becomes utterly relaxed. When my 15-minute session is over, I'm saturated in sweat.

Washed, I slip into the pool and rest my head on the tiled edge. Somewhere between sleep and a daydream, I check in with my body. I'm hot but not uncomfortable; I'm lightheaded but not dizzy; I'm relaxed but not tired. I let out the biggest sigh of my life, and with it, the enigmatic veil of onsen starts to fall away. I'm sans clothes in a small pool of piping hot water - with other naked men - and to be frank, I couldn't care less. The ritual begins to make sense.

But the science and geography are best explained at the Jigoku Onsen Museum across a dazzling array of exhibits and interactive chambers. I witness an audio and visual journey of one raindrop over a 50-year cycle until it is released as spring water. One display poignantly points out, "Hot spring water changes into a unique form depending on the path it travels. The water you encounter on your journey is something that happens only once and never again." The tour finale is a big screen which retracts to reveal a window overlooking an immense steam vent just outside - there the whole time. It's a vivid way to conclude my onsen education.

The coursing Hanaawaseno River is pleasant white noise as I take in the valley view at Yunohira, high in the hills west of Beppu. The traditional and comfortable ryokan lodging at Takaosou Inn means a bedroom window with sweeping views of the valley. I tuck into a hearty teishoku set meal before a dip in the property's private outdoor onsen. Alone, the experience takes on another persona completely. The formality of bathing naked after a full evening meal is undoubtedly odd to the Western psyche. But as time passes, I feel myself increasingly ready for bed. In fact, the brief 20-minute soak - staring up at the stars - would provide an underpinning for one of the most restful night's sleep I've had in a long time.

The following day, I go further south to Nagayu, driving over emerald hills and rolling pastures as dark clouds appear. I overlook the first one, and perhaps the second, but certainly the third Bavarian-style building catches my attention. The architecture is a respectful, yet unusual, nod towards the town's European sister city, Bad Krozingen, in Germany. Japan's knack for replicating other cultures is, after all, forever impressive. However, my assignment isn't buildings or scenery; as a newly self-proclaimed spa hobbyist, I've come to sample something unique: carbonated onsen. A consequence of nature and local topography has surprisingly created spring water rich in sodium, magnesium ... and natural fizz.

At Lumine Onsen - behind a black and white wooden exterior that would look more at home in the Alps - there's a discernible metallic smell to the water as it pours into the small tub. I lower myself in and, within seconds, I can feel millions of tiny beads cling to my skin. These springs are known to have slightly cooler water, improve circulation and lower blood pressure; I feel as if I could sit and let the bubbles do their magic for hours. It's the tenderest of massages. A crack of thunder overhead is followed by a slow chorus of raindrops on the garden outside. Perhaps I will - all in the name of learning, obviously.

Ashi-yu: These communal foot baths with bench seating are often found on the streets and have free-flowing spring water. It's a great way to rest and give your feet some love.

Sunaburo: In robes, guests are encouraged to bury themselves in geothermal sand pits for extended periods and experience a deep, slow sweat to improve circulation.

Doroyu: Extremely rare and hard-to-find mud baths where entire pools are filled with hot mud or hot water baths and mud is provided from a trough to be applied to your skin. Beppu is one of a few places in Japan where this type of onsen can be found.

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Getting there: Airlines such as Jetstar Japan, Peach and Solaseed Air regularly service direct flights to Beppu from Tokyo Narita, Tokyo Haneda, Osaka and Nagoya.

Staying there: Rates at the Amanek Yula-Re vary greatly across room types and seasonal promotions are offered for booking in advance. amanekhotels.jp/beppu

The Takaosou Inn's prices start at about 13,000 yen ($137) for solo travellers, including dinner and breakfast. peraichi.com/landing_pages/view/Takaosou

Onsen time: Prices to visit onsen range from 400 to 2000 yen ($4 - $21) depending on the size, quality and amenities offered. There are often opportunities to rent towels and buy specific soaps and shampoos. The expectation is you will wash yourself thoroughly before entering the onsen pool. In doing so, take your time. If the water is very hot, ease yourself in slowly. It's recommended you don't exceed 30 minutes for any one session.

Explore more: japan.travel/en/sg/guide/oita/

Alex Mitcheson was a guest of Tourism Oita.