Rusukaina

Rusukaina is an island in the Calm Belt that is to the northwest of Amazon Lily. It is a harsh island that has forty eight seasons with the season changing weekly. There are several different species of dangerous animals on it. Boa openly stated that the island is dangerous even by Kuja standards. A country supposedly used to exist on this island, but its people could not survive against the extreme environment and died out. According to Silvers Rayleigh, when Monkey D. Luffy arrived on the island, there were over five hundred beasts living there he would not be able to defeat.

History
Rayleigh took Luffy here to train him in the use of Haki for two years. After the first year and a half, Rayleigh left Luffy on his own after he taught him the basics of Haki. After another half a year, Luffy became the "boss" of the island, and even befriended some of the animals (which Luffy considered a mistake, as he could not eat them). He departed when the time came, with the Kuja Pirates coming to pick him up.

Trivia

 * "Rusukaina" means "Anybody home?"