The Crystal Caves of Ossus were a series of deep cave systems within the Eocho mountains. Within the caves, Adegan crystals were plentiful and prized by the Jedi Order for their use in lightsabers. During the Order's time on Ossus, Jedi Masters often sent their Padawans into the caves as part of their Trial of Spirit. Once inside the caves, the vergence there created a sense of unease in the Force, and made those who entered feel as if they were being watched. Many Padawans claimed to have dreamlike experiences wherein their deepest vulnerabilities were laid bare. In the rush to collect as many Adegan crystals before the Cron Supernova irradiated the planet, a group of Jedi became trapped in the caves by a collapse in part of the tunnel. Surviving the cataclysm on the surface, these surviving Jedi were the progenitors of the Ysanna people that would come to roam the wastes of Ossus and protect its ruins.