Here are some of the most common types of eruptions: In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from a vent or line of vents (a fissure) at the summit or on the flank of a volcano.
The underscore sign is mainly used to show a space where a space is not allowed, such as in internet usernames, email addresses and some computer programs. The underscore looks like a dash below the letters (_).
These are all part of the language, and, as such, can be found in any text that properly describe the language, such as Scala Specification(PDF) itself. The last one, the underscore, deserve a special description, because it is widely used, and has different meanings depending on the context.