The tradition of sharing the spring with visitors has also spanned generations: The tribe ran a succession of bathhouses, then opened the Spa Resort Hotel on the site in 1963. When this multimillion-dollar, 131-room Modernist complex debuted, it became the place to take the local waters. Indeed, it was the only place, because it retained exclusive access to the Agua Caliente spring, the sole option in town. So when everything in the complex except the casino closed in 2014, anyone hoping for a warm mineral soak in Palm Springs was stuck on dry land.

The tribe had determined that the 1950s-era water collection system needed enough repair to require the spa’s demolition — a catalyst for rethinking the entire site. “We’re talking about one of the most important sections of the reservation,” Mr. Milanovich said. “We had to make sure it would be protected for future generations.” The best way to do so, tribal members and leaders concluded, was to create experiences that were educational and celebratory of Cahuilla heritage.

After nearly a decade of work on the site, and one of the most significant Indigenous archaeological recoveries in the country — thousands of artifacts unearthed; thousands of years added to the local historical record — the new Spa at Séc-he opened on April 4. And by late 2023, a neighboring museum will delve into the excavation as well as the tribe’s history, culture, language and more.

Though right in the heart of Palm Springs, Séc-he (pronounced SEH-hee, or SEH-khee) is only one of numerous spring-fed spas in the region. About 10 miles away, the town of Desert Hot Springs — with its own famed aquifers and soaking sites — is bubbling back to life after declaring bankruptcy in 2001 and narrowly avoiding a repeat in 2013. Since becoming the first place in Southern California to legalize large-scale medical-marijuana cultivation in 2014, however, the community is rebounding, new retreats are opening and old favorites are expanding.

Here’s a guide to total immersion in Palm Springs and Desert Hot Springs.