Around 10 minutes by car from Shizuoka Station is a nondescript, three-story building, that you may be surprised to hear is considered a sauna mecca by fans of the Fennoscandian heat rooms.

Once primarily catering to a middle-aged, male clientele — many of the tattooed yakuza variety — Sauna Shikiji has seen the number of younger customers soar over the past several years as hitting the hot box became a fad, stoked by celebrities and trend-conscious women calling themselves "saunners" fervently embracing the activity for its purported health benefits.

"Things began changing over the past five years or so," says Mikie Sasano, the daughter of Sauna Shikiji's current owner and a public relations consultant, spa and restaurant producer. Together, the Sasano family transformed what was once an unremarkable facility in the suburbs of Shizuoka Prefecture into one of the nation's most popular sauna spas. The key to its success was Sauna Shikiji's unrestricted access to mineral-rich natural groundwater it uses in everything from cooking and laundry to its famed cold bath.