It is a huge pit that water flows into, yet the bottom is dry. There is a village where the citizens live by the coast and there is also a ship graveyard, full of destroyed ships. When the village ends, there is a huge mountain where the treasure of the Ocean's Navel is.
The villagers lived peacefully until Joke and his crew came and destroyed everything, twelve years before the current storyline. After that, the crew went to the mountain of God where the hoped to find the treasure that would make their wishes come true. But when they reached the treasure room, the crew members betrayed their captain and awakened the treasures guards, three huge monsters that kept terrifying the islanders for ten years. When the Straw Hats reached the island while searching for the treasure, they killed the monsters which made Captain Joke's body revive. But Hamu killed the pirate with the help of his friends and the monsters started guarding the treasure once again.
↑One Piece Anime — Episode Special 1, Before the timeskip, Meroie states that Joke and his crew arrived ten years ago.