The claim to fame in Thermopolis, Wyo., couldn't be clearer. Huge white letters atop a red butte shout it out: "World's Largest Mineral Hot Spring." The arrow points downward to Big Spring, which supplies more than 2 million gallons of 129-degree water from the earth each day.

A visit to the springs is a perfect way to cap off a trip to Yellowstone; here you can slip into a beguiling milky blue pool reminiscent of what you see (and smell) throughout the national park. Thermopolis even has its own colorful terraces like those at Mammoth Hot Springs.

Shoshone and Arapahoe Indians signed over Thermopolis' Hot Springs in an 1896 treaty with the assurance they would always be free to the public. That was especially welcome in the early 20th century when "taking the waters" was considered curative and healing.

You can still soak for free in an indoor pool or a square pool behind the 1966 state bath house with views of the terraces. The catch is you only get 20 minutes every two hours due to the temperatures. You also can soak or play the whole day at Star Plunge and TeePee Pools on either side of the bathhouse for $5 to $10. Some mineral water also gets diverted to outdoor spas at the Days Inn (1-307-864-3131) and Best Western Plaza Hotel (1-307-864-2939), each within the state park.

The springs' cooked-egg aroma of sulfur is noticeable, but we decided it wasn't any more overwhelming than a strong dose of chlorine. Our fellow soakers swore the hot springs' 27 minerals help everything from achy joints to rehabbed shoulders. It did feel surprisingly therapeutic -- relaxing like an end-of-the-day bath, invigorating like a morning shower. Mostly, it just felt good.

Lisa Meyers McClintick