many of maps are randomly generated. while, in many cases, the first map in a storyline mission is pre-set, the "extensions" to the map are often randomly generated. the randomly generated maps creates the potential to have vastly different situations even when replaying the same map. within the game, there are two separate types of extensions: "locked" and "secret". while both extensions can be revealed by throwing something out of bounds into the extension, "locked" extensions can also be unlocked by destroying a specific unit on the field, which is marked with an icon saying "key".