(I) Volcanic eruptions in populated regions are a significant threat to people, property, and agriculture. (II) Most volcanoes have steep sides, but some can be gently sloping mountains or even flat tablelands, plateaus, or plains. (III) The volcanoes above sea level are the best known, but the vast majority of the world's volcanoes lie beneath the sea, formed along the global oceanic ridge systems. (IV) More than 1,500 above-sea volcanoes have been active during the past 10,000 years, 539 of them erupting one or more times in recorded history. (V) On average, 50 to 60 above-sea volcanoes worldwide are active in any given year; about half of these are continuations of eruptions from previous years, and the rest are new.