The first written record of the hot springs at Chico was in the diary of miner John S. Hackney, dated January 16, 1865. Then in the late 1890's the hot springs flowed into two wooden tubs about four feet deep inside a small wooden building located where the hot springs come to the earth's surface. The Main Lodge of Chico Warm Springs Hotel was opened in 1900 and a larger hot springs pool was built. The pool was 44 feet in diameter and six feet deep, and there were also private baths and baths for ladies only. By 1902 the "Plunge" was doubled in size and there were two large oval shaped pools, one six feet deep and one nine feet deep, which were completely enclosed in a wood building. Between 1917 and 1919 the "Plunge" was changed again - one large rectangular pool took the place of the oval pools, a pool lobby was built (the current saloon) and additional lodging rooms were built as a second floor over the top of the pool lobby. The "Plunge" and building housing the pool went through further changes prior to the 1950's. In May of 1957 the rounded roof over the "Plunge" collapsed on approximately 70 swimmers. Only two swimmers had minor injuries while everyone else walked away unharmed. The open air pool became popular with soakers and swimmers and the roof was never rebuilt over the large pool.

The "Plunge" was popular not just as a swimming pool, but as healing waters in the early days of the resort. There was advertising promoting the curative powers of the natural mineral hot springs pools for everything from kidney troubles to blood disease and skin disorders.