Bedrock

Bedrock is the material at the core of all islands, typically at a depth of around 50 meters. It is a pitch black rock with light grey marbling. This material is completely impervious to all but the most devastating of attacks. If it is damaged, the magical seal which holds it aloft is broken and the entire mass shatters, dropping the island from the sky. This is not a fear for most inhabitants, as the bedrock is covered by a protective layer of dirt on almost all islands, and even exposed bedrock is not a substantial risk as it is so durable that all but the most powerful creatures are incapable of summoning a strong enough attack to destroy it.