It is a crypt of Miyagusuku family, located in the hilly region of Okinawa. The gravesite contains the tomb of George's wife and daughter. When Saya went in hibernation after the Vietnam War, George kept her cocoon inside the crypt. After his family died, he wanted to commit suicide but Saya's heartbeat gave him the inspiration to live on. Saya's awakening gave George a new turn and he adopted her as his daughter.
George took Saya to the crypt before he was attacked by chiropteran. There he told her how he found her and what had happened in his life before. After George's death, he was also buried there and Saya, Kai and Riku visited the place one last time before the three of them set on a journey to hunt down Diva.
Saya, after defeating Diva, goes back to her 30 year hibernation cycle and Kai takes her to the crypt once again. There she remains till the end of the anime where Kai returns, years later, with his adopted daughters (Diva and Riku's children) to visit Saya and it seems that Hagi has recently been there too.