Spas with thermal pool

A resort spa in Tucson's Starr Pass area with a saline lap pool, sauna and fitness center, plus signature treatments from $159 for hotel guests.

North Scottsdale day spa with massage, facials, sauna, steam and a thermal pool, plus private suites for couples and groups.

An adults-only wellness retreat on Ubud's Ayung River offering multi-day healing programmes, thermal circuits and plant-based cuisine by reservation only.

A hotel spa at Eden Roc Miami Beach in Mid-Beach with sauna, steam room, cold plunge and thermal circuits, open to resort guests only.