Spooni-Nui is not an island, but a Metal Structure built in the the middle of the protrodermis ocean built by a crazed Matoran. The Matoran made it in the shape of a spoon because he believed that his hero, Toa Ghouloki - Toa of Spooks was in fact a Toa of Spoons. So it was built in his honour with that one big mistake. Many Matoran didn't want the construction of Spooni-Nui to go ahead saying that it was craziest plan they'd ever heard of, and heck it was crazy but the construction sill went ahead and has become majorly successful.