When we think of volcanoes, eruptions, lava, and smoke-filled air come to mind—all occurring on land. Most people are surprised to learn about the prevalence of underwater volcanoes on our planet. Because the lava and smoke spilling out of an active, underwater volcano is contained by the ocean, people generally do not take note of these eruptions. However, the largest underwater volcanoes are capable of creating huge tidal waves, threatening coastal communities. The main idea of the passage is that: