In a time before anyone, even those of a less mortal stock, could possibly hope to remember, the ground seethed and shifted. A large valley was flooded with lava from an ancient volcanic eruption. Bubbles formed in the viscous body of the rapidly cooling lava and created a large system of tunnels and pockets of air. Later, when this former valley was settled to become the site of a wondrous Capitol of the afterlife, some of these chambers were excavated into what would become the massive sewage and supply system of the Sereitei.
However, a percentage of the pre-formed chambers were either never found or were never connected to the system. An underground river eventually flooded one, and the positioning couldn't have been better. That flooded chamber was set deep enough that it became a hot spring. Of course it was eventually discovered and to make a long story a bit shorter, it became very popular for the collective residents of the Sereitei.
It was very rare to see members of the Ichibantai at the hot spring. Many assumed it was because the squad was too busy or disciplined to waste precious time in such a place. While that may be true, it wasn't a causal variable. You see, not only was that hot spring close to the Ichibantai's barracks, but it also possessed a host of branches. One such branch found its way to a chamber directly below the barracks.
The cave is almost obnoxiously large, but the branch spring is shallow and warm. Ventilation keeps taking a dip from being deadly and the relative darkness allows some semblance of privacy. A few overhead lights were installed but the spring has two main alcoves to help with gender division. Luckily, the spring feeds out of the chamber through a crevice in the far corner where the water deepens.
The general agreement is to keep the whole thing a secret, though a few members have had the gall to invite a few friends. Most members tend not to use it because, despite the vents, the air can be stifling and very little has been done to it which makes the entire experience more rugged-seeming.
