In August 2006, the crew of the yacht Maiken, sailing out of the Vava'u island group, encountered a remarkable "stone sea." The phenomena they spotted were a series of pumice rafts resulting from a nearby eruption, one that was forming a new volcanic island in Tonga.
Before when an new island appeared due to volcanic eruption on international waters, I believe you could claim the island your own and declare independence if you found it first. Essentially having your own small country or micronation.
But ever since Sealand, I don't think thats possible anymore, laws changed.