It is a place belived to be where yagara bulls go when they die. It is believed by many Water 7 civilians to be a legend. The only one who believes it to be real is Abi. She asked Luffy and Chopper to help her find her family pet yagara, Aobire.
When they found him he was dead and he was below Water 7 (which is where Tsui Village is). This is the place where Aobire died by trying to save a baby yagara.