Source: https://www.flickr.com.Postcard scan by Jordan Smith. License: All Rights Reserved.

World's largest natural warm water swimming pool, fed by springs of medicinal qualities, with a water temperature of 90 degrees. Inflow of 5,000 gallons per minute, complete change of water 16 times daily, insuring perfectly clean, fresh living water at all time.

The photo on the postcard is probably by Don Mueller, Scenic West Photography, who took a similar shot of the other end of the same building. This card was produced around 1960. The building was rebuilt in 1970. The attraction is one of the oldest tourist destinations in the Black Hills. Here's how it looked when it opened in 1890.