The Volcanic Island is a unique biome. It is a large mountain made out of Ash Blocks and lava. Caverns are found more often here in this biome. Survival in this biome would be difficult due to the lack of trees, impossibility of farming, and lack of passive mobs.
On the other hand, finding one can be useful due to the abundance of Ash, which can be made into coal. As of 1.8, it is now an ocean subiome.