The term for a former volcanic island that has sunk below sea level is a seamount. Seamounts are underwater mountains formed by volcanic activity that can eventually become submerged. Many seamounts are remnants of once-active volcanoes that have eroded over time. These formations provide unique ecosystems that support various marine life. Additionally, seamounts are of great interest to researchers studying plate tectonics and the biodiversity of oceanic habitats, contributing to our understanding of geological and ecological processes.